The organization of Jehovah's Witnesses is a cult: dangerous, divisive, abusive. It's a cult that has destroyed thousands upon thousands of lives. Perhaps you can get your bf's sister's attention with identification of the JWs as a cult. And of course they've been so nice to her. The "love-bombing" so common to Jehovah's Witnesses at this stage is also common to the Moonies, the Scientologists, the Mormons, the Seventh-Day Adventists, Hare Krishna, Ekankar, and every other cult at this stage too.
If your bf's sister will acknowledge that if it walks like a duck, quacks like a duck, looks like a duck, has feathers like a duck, swims like a duck, it must be a duck. That said, please try to have her examine the following characteristics of every cult. You might have one or two of these characteristics and not be a cult. But not all (and there's more than I have included here):
1. The cult has an infallible leader offering an extravagant hope.
The Governing Body of Jehovah's Witnesses is just such an infallible leader. Whatever the GB says is regarded as coming from God, even if it is in direct contradiction of something they said last week, last month or last year. And you must accept it. If you do not accept it, whatever it is, no matter how it outrages your own sense of justice, mercy, compassion or fair play, you will be disfellowshipped because you refuse to accept it. I've seen it happen many, many, times. And this is the way you loose your family to the cult, or they loose you. The GB has stated that it is "the only channel of communication God is using" to communicate with people on the earth today. And you must believe it. And preach it to others.
The Extravagant Hope part of this is everlasting life on a paradise earth. Soon. In fact within your immediate lifetime. This is something like the fifth or sixth time they have prophesied the "end of the world" in the last hundred years, a fact they don't discuss, and deny when confronted. The last date the world was supposed to end was October 10 (or close by), 1975 in the early daylight hours. JWs sold their homes, took out huge loans they knew they weren't going to pay back, postponed marriage and children, etc., etc.
2. Questioning, doubt, and dissent are discouraged or even punished.
As noted above, you must accept as coming directly from God all teachings of the GB. If it's in the Watchtower or the Awake, or any JW publication, you've not only gotta believe it, you've gotta preach it door-to-door.
3. Mind-numbing techniques are used to suppress doubts about the group and its leader(s). One of these techniques is to keep you very, very busy in cult business all the time with no time to think and reflect on what's really goin on.
There are five meetings a week. You will be expected to be at every one of them. Plus you will be expected to participate in the door-to-door work for at the very least ten hours a month. You will also be expected to be at an assembly, large or small or maybe both, during the summer months. This usually means travel to a large city and sitting in hot, hard bleacher seats for up to ten or twelve hours a day for four or five days. If you decide to skip it and go on vacation with your husband, the elders will know about it and you will explain your private decsions to them. Any less than this and the "love" they show you will disappear and you will be "marked" as spiritually weak.
4. The leadership dictates sometimes in great detail how members should think, act, and feel (for example: members must get permission from leaders to date, change jobs, get married; leaders may prescribe what types of clothes to wear, where to live, how to discipline children, and so forth).
JWs, especially the women, must wear what is approved by the society and/or the elders in your congregation. In my congregation this meant no pants suits, no pants of any kind, nothing considered "immodest" by the elders to include hair styles, fingernail polish, jewelery, etc for the women. It also meant no steel-rim glasses, granny glasses, bell-bottomed slacks, wide neckties (in fashion at the time), colored shirts (white only) and certainly you couldn't be a "fashion plate" like me, always wearing the latest fashion. Your behavior is examined under a microscope, the way you raise your children is considered fair game for the elders to dictate. In fact, the way you live down to the smallest detail is fair game. And that includes the intimate, nitty-gritty details of the way in which you engage in sex with your husband. There are many no-nos. No oral. Certainly no anal. Nothing the slightest bit kinky. Preferably sex is for having children and is best conducted in pitch black darkness in the missionary position and is to be enjoyed as little as possible. And you can and will be questioned, alone, in minute detail by a group of three men if they get the slightest opportunity. This is mainly to satisfy their voyeuristic proclivities. I've seen this happen.
I lived with my "elder" uncle while I was in college in Tennessee in the late 60s. He would set his alarm and get up in the middle of the night to go spy on his bible studies (like you) as well as members of his congregation.
5. The group is elitist, claiming a special, exalted status for itself, its leader(s), and members (for example: the leader is considered the Messiah or an avatar; the group and/or the leader has a special mission to save humanity).
"As noted, JWs consider themselves to have "The Truth" and are the only channel of communication God isusing..."
6. The group has a polarized us- versus-them mentality, which causes conflict with the wider society.
This will become more and more apparent as time goes on. The world is made up of two groups: Jehovah's Witnesses and everyone else. Everyone else is regarded as "worldly" and is considered dominated by "Babylon the Great, the world empire of false religion" which in turn is controlled by Satan and his demons, which means everyone not a JW is controlled by Satan including your husband. Association with the world is considered a giant no-no, and you can be "marked" for doing it. By the way, your bf is worldly.
7. The group teaches or implies that its supposedly exalted ends justify means that members would have considered unethical before joining the group.
Jehovah's Witnesses consider lying an acceptable technique in its spiritual warfare. And have said so in print. You will not be allowed to take a blood transfusion. If you do, you will be disfellowshipped, shunned. It's been the JW position on blood for decades. However, the JWs in an attempt to be accepted by the government of Bulgaria as a recognized religion, have testified in court, under oath that they strike no position on the blood transfusion issue. So when you ask JWs who are working with you about any of these things, you are very likely to be lied to.
8. Cults always have some particular group hatred. With the JWs, that would be people who've had enough and left, apostates. Apostates are to Jehovah's Witnesses what the Jews were to the Germans in WWII. Someone to blame for everything and someone to get together and engage in a group hate and persecution. I'm certain that if the JWs could get away with killing us in the ovens, they would.
These are just a few of the characteristics of a cult. And I have been totally truthful. I was born into this cult. I would have never joined. But I stayed in long enough to have been very, very hurt by it. I lost members of my family and friends for up to 16 years. My best friend from college still hasn't spoken to me since I left. That was almost 24 years ago. And I miss him so much till this very day.
I could go on and on and on, but I'm not going to. All you have to do from this point is to get on a search engine on the internet and type "cult characteristics" or "Jehovah's witness cult" or some other phrases like that and see what you get.
So yes JWs are without a shade of a doubt a cult. Don't be surprised that they treat people on the way in very well. They want something from your bf's sister: her very life. They want to turn her into a magazine selling, meeting going, hand-raising, comment-making, head-nodding, empty-skulled zombie. Once you get baptized, that "love" will come to a screeching halt. Count on it.