Here's some interesting info I found. I can also relate to your friend's sister, as I have had the same happen to me.
. http://watarts.uwaterloo.ca/~acheyne/incubus.html#p&b
"Among the best-known experiences associated with sleep paralysis is the sensation of a strong pressure on the chest or back. Most often it is on the chest since sleep paralysis is also associated with lying in the supine position. Often it seems that there is someone or something sitting on one's chest. Sometimes this entity may also be experienced as choking the victim, or just pushing forcefully on the chest, or even biting into the shoulder, or twisting limbs. One of the most evocative accounts of the sense of the incubus (incubare: to lie upon) / succubus experience is provide by the writer Guy de Maupassant in Le Horla.
Experience of Pressure and Breathing Difficulties
One of the traditional experiences associated with SP is that of extreme pressure on the chest, the dominant theme of the incubus or "Old Hag" attack (Hufford, 1983). Although the classic hallucination is of a crushing weight or presence on the chest, the pressure may be felt on the back or side of the torso. Sometimes the experience is simply of a pure pressure on the chest or torso without further elaboration.
I experience pressure on my chest.
I will feel a strong, heavy pressure on my torso, and occasionally my limb will feel like lead.
Pressure on my chest...every episode
Usually I feel a strong pushing feeling against my entire body
Sometimes the intensity is sufficiently intense that the language becomes somewhat stronger,
"All my SP has contained extreme pressure, crushing sensations on my body."
Sometimes the pressure or pushing on the body is felt to be so intense that the person feels as though s/he is being pushed/pulled into the bed.
Feel that my whole body is being pulled, as by a magnet. Sometimes just my feet, or an arm; sometimes feel I am being pulled into the bed.
Its like the pull of gravity from underneath increasing- can't really describe it.
I recall one experience in which I felt as if I was sinking into/through my bed.
I once thought I'd done a 360 degree turn in my bed, and once felt as if I were pulled out of the bed. I thought I was nuts.
The sensed presence is sometimes associated with these bodily assaults.
The unseen presence was the cause of the experience. It gives you the feeling of something really evil and it is always frightening
. . . I never see this being . . . It felt like this thing wanted to kill me . . . My eyes are open during these experiences and it makes it more frightening because I cannot see what it is that is holding me down.
Sometimes the presence seems to attack the victim causing feelings of pressure on the chest or back depending on the person's position.
" Sensed an evil presence watching me, which I felt was responsible for my paralysis." "I feel a very strong force pushing down on my chest, and pulling me down towards the floor. I also feel a presence in my mind (like something sinister or evil) that is trying to draw me into an extremely deep, permanent sleep. I feel that if I succumb, I will never wake up."
Some respondents go beyond the description of pressure to make further interpretations about the source of pressure. It may be experienced as “something sitting on my chest, but recently I've been embraced from behind” or even more actively and specifically as the “pulling and clawing of . . . dog.”
The pressure on the chest is often associated with perceived breathing problems.
I never feel like I can get enough breath. I tend to take quick shallow breaths.
This almost always occurs, as far as I remember, with an episode of apnea or labored breathing through a narrowed airway while sleeping on my back. I normally do not snore but my husband has reported that I do while on my back. After awakening from one of these unpleasant experiences, I often enter consciousness barely in time to hear a snort made by myself, or to feel myself gasping for air. While this paralysis occurs I can sometimes feel my chest struggling to get air as my throat closes slowly. I try to move a limb to awaken, but of course it doesn't work and I have to wait until I awaken on my own, perhaps because my blood oxygen level has fallen enough to awaken me.
Whereas the sensed presence is often said to be associated with a nameless terror, sensations of pressure and difficulty breathing are more definitely associated with thoughts of impending death. Specific references to the prospects of dying from suffocation.
...feeling like I am going to die of suffocation.
I felt I was suffocating, and death would result if I don't wake up.
There were times I thought that I would die because I felt that I would never get my breath back. It is a horrible feeling.
A couple times only, as I felt I was dying (suffocating) I felt myself 'leaving my body ' and then it ended.
I felt I was suffocating, and death would result if I don't wake up.
There were times I thought that I would die because I felt that I would never get my breath back. It is a horrible feeling.
Always have difficulty breathing as if the life/breath is being “pressed” out of me, suffocating me, to the point of feeling “This is it, I'm a goner!”
A couple times only, as I felt I was dying (suffocating) I felt myself 'leaving my body ' and then it ended.
The difficulty breathing is sometimes experienced either along with, or perhaps as, a pressure on the chest. This pressure is interpreted as pressing or pushing of air out of the lungs. Again this may be attributed to some agency.
As if a large animal is sitting on my chest, making it difficult to breathe and if I do not wake up I eventually feel as if I'm being choked. Sometimes I feel pressure on the lower extremities, sometimes just on the bed. Usually it's on my chest.
Sometimes it's very hard to breathe as if someone is sitting on my chest.
The interpretation may then expand to a feeling that, in addition to pressing on the chest something is choking the individual.
"I would feel like someone was sitting on my chest, trying to choke me."
Sometimes the pressure/breathing difficulty is compared to or attributed to pressure or constriction by bedding.
On a few occasions, felt like a sheet pulled tight on my frame... so I couldn’t breath or move.
My chest felt great pressure but it seemed rather internal than external. The paralysis was like being under a heavy wet blanket.
I feel a very strong force pushing down on my chest, and pulling me down towards the floor. I also feel a presence in my mind (like something sinister or evil) that is trying to draw me into an extremely deep, permanent sleep. I feel that if I succumb, I will never wake up.
The pressure may be felt in various parts of the body or even over the whole body.
The pressure is always on my entire body.
Sometimes I feel extreme pressure on my head, which is very painful but [it] goes away as soon as I snap out of it, other times I feel that I am being physically assaulted by something.
Whenever I wake up in a paralysis, I feel pressure on my midsection, as if someone were sitting on me.
Sometimes like the ceiling was coming down on me squeezing the life and breathe from me but this is very hard to distinguish from being abused and my episodes with sleep disorder they are mixed together here...
As in the case of the presence the pressure may be associated with unnatural agencies and/or life threatening assault. That hallucinations might be associated with apnea during SP is consistent with the finding of Hobson, Goldfrank, & Snyder (1965) of a correlation between respiratory apnea and similar dream mentation. Thus, sometimes when difficulty in breathing is associated with the sensed presence it is interpreted as an attack of the presence.
During my first experiences I believed a ghost was sitting on my back, pressing me down! . . . like an enormous force pressing down on me.
Commonly I will wake from a dream and sense it's thick presence. Then it will rush at me. Climb on my back and pin me to the bed.
Although it wasn't a pressure on my chest but the hallucination of "someone" holding my head down with a pillow.
Incubus and REM
Characteristics of REM respiration include shallow rapid breathing, hypoxia, hypercapnia, and occlusion of airways (Douglas, 1994). Both tidal volume and breathing rate are quite variable during REM, and because of paralysis of the major anti-gravity muscles, thoracic contribution to breathing is lower during REM than during NREM sleep (Douglas, 1994). Moreover, because individuals are conscious during SP they may sometimes attempt to breathe deeply when tidal volume decreases, just as they attempt other voluntary movements. When they find their attempts to control breathing volume and rate are unsuccessful, they will feel a sense of resistance possibly interpreted as pressure. This is consistent with the finding that the pressure on the chest is positively associated with perceived breathing problems (Cheyne, Newby-Clarke, & Rueffer, 1999; Cheyne, Rueffer, & Newby Clark, 1999). In addition, increased airflow resistance because of hypotonia of the upper airway muscles and constriction of the airways would result in feelings of choking and suffocation leading to panic and strenuous efforts to overcome the paralysis. This sequence of events is consistent with a report by Hobson, Goldfrank, & Snyder (1965) of a dream immediately following apneic respiration during REM in which the dreamer reported being choked in a dream play. Finally, because of the paralysis, the absence of dampening proprioceptive feedback following execution of motor programs associated with struggle may further lead to painful spasms (Ramachandran, Rogers-Ramachandran, & Cobb, 1995). In summary, we suggest that the incubus experience results froma cascading series of events initiated by complex respiratory conditions leading to experiences of thoracic pressure, choking, and physical assault with associated pain.
SP and Tonic Immobility
It has been argued and evidence has been presented to support the following claims: the sensed presence appears early in the sequence of HHEs; it is intimately connected with an intense fear, which is often better described as terror; and it may condition or modify the character of other HHEs, especially visual and auditory hallucinations. We speculate that the sensed presence is experienced as an undetermined source of threat giving rise to search and interpretive efforts to find and identify the source of threat. We argue further that, when the SP episode persists, the threat of this presence become realized in increasingly elaborate interpretations of other HHEs, initially arising either endogenously in oculomotor activity, middle ear activity, or exogenously via ambient noise and shadows. Several features of the way in which respondents characterize the presence are consistent with the notion of the cluster of presence, fear, auditory and visual hallucinations are related to mechanisms underlying responses to predation. The presence is sometimes described as a monitoring one, akin to predator stalking its prey. Furthermore, a malign or evil intent is ascribed to the presence. What more compelling source of terrifying evil could there be than an unknown creature that seeks our annihilation? Alternatively, or subsequently, bodily sensations of pressure and respiratory difficulty, may be interpreted as a full-fledged attack. We believe there are sound neurophysiological grounds for supposing that the sensed presence associated with SP is fundamentally associated with fear and a response to predation. SP may be seen to have many parallels with to a common antipredator response referred to as tonic immobility (TI)
Ratner (1975) describes a sequence of relatively fixed action patterns prey animals engage in when stalked, chased, seized, and attacked. TI, or death feigning, is an escape strategy of last resort, induced by fear, restraint, or pain such as that produced by being attacked and seized by a predator or rival conspecific. When an animal is seized and restrained it may initially struggle and counterattack. Following a brief struggle many prey animals quickly lapse into a cataleptic state. The cessation of struggle and the profoundly unresponsive immobility may sometimes cause the predator or other attacker momentarily to release the victim. Thus, TI may be an adaptive strategy for coping with otherwise hopelessly inescapable situations such as being caught and held by predators or when reaching the point of exhaustion in flight or fight situations. TI has been proposed as a model for several disorders including catalepsy and catatonia (Gallup and Master, 1977; James, 1890) and there are a number of reasons that it might be an instructive model for sleep paralysis. There are numerous and striking parallels between SP in humans and TI in a wide variety of animals. Aside from the motor paralysis, the association of external monitoring, threatening presence, bodily sensations associated with being attacked, mauled, and transported, as well as fear itself, are common features of SP and tonic immobility. For example, TI may be induced by restraining the subject in a supine position. The duration of TI is substantially increased in the presence of an image of a predator or even a pair of disembodied staring eyes (Gallup and Master, 1977). In addition, the reported subjective experiences of the threatening presence and the crushing pressure on the chest are strongly suggestive of the predator/attacker that seizes and holds down its victim. At the very least the paralysis in the supine position might predispose one to interpret the context in life threatening terms. These may then lead to a series of internally consistent interpretations concerning the meaning of events of either exogenous or endogenous origins. In this sequence of events one of the first might be to locate and external source of threat, an external threatening presence. Moreover, only pressures applied to the midline of the dorsal surface at the nape of the neck significantly potentiated the duration of TI and cling catalepsy (Meyer, 1990). These physical manipulations would certainly produce sensory experiences of dorsal or ventral pressure around the thorax and neck regions such as are reported in the Old Hag experience. However, it is likely that in the normal ecology of animals subject to predation predators are the likely sources of the positional and tactile elicitors of the condition. It is especially suggestive then that the phenomenology of SP conjures up an image of an external threatening presence. Both the quantitative and qualitative analyses reveal that the felt presence, felt pressure on the chest or back, and fear are common associates of the sleep paralysis experiences (Cheyne, 2000; Cheyne, Rueffer & Newby Clark, 1999). These are just sorts of experiences one might expect to be associated with a state related to that of tonic immobility. The strong association of felt presence and fear and terror as well as the perception of the presence as posing a threat is certainly consistent with the notion of the association of immobility states to flight and fight situations. The vagueness and insubstantial nature of the auditory and visual hallucinations associated with sleep paralysis is likely simply a result of the anomalous endogenous neural, or ambiguous exogenous, origins of this state. In any case, the origins of sleep paralysis in the tonic immobility response appears promising as a partial explanation for the major phenomenological aspects of sleep paralysis."
UB, the site has many other references that you may want to look at.
Hope this helps!
edited to fix forum codes