lotus you have a plateful and a half with relatives, a husband dealing with PTSD, 2 small children and goodness knows what else. In short you are overcommitted and some or all of these things is helping to fuel the issues within your marriage. In addition just as you are dealing with these things your husband is too it is not a happy mix.
Relatives how long are they staying and what process is in place to move them on? It sounds callous but having an end date is essential. Staying until they die is never a good option unless you love them very much, they are extremely old and you have the time to care for them properly. If your relatives do not meet this criteria and they are adults then they need to be aware of the temporary nature of their accommodation with you. You need space in your home to deal with your own relationship with your husband.
My MIL is a challenging lady, she had an abused childhood and was an abusive parent herself. She is very high maintenance and she lived with us for a time until she could get herself settled. I never allow things to escalate and refuse to play the game. If she is sulky and angry I just ignore it and treat her with warmth and affection as if it just isn't happening. Because I don't fuel her behaviour it is hard for her to sustain it. One day she said to me you treat me dreadfully. I responded with "Be grateful I treat my own mother much worse." I have to see past the issues with her and realise that she is not entirely responsible for how she is. I am a relatively calm person so it helps I show her affection and love even when she frustrates me and try to listen to her concerns and issues. Over the years I have learnt what are her trigger topics and avoid them or diffuse them but it has taken time. My advice is as soon as she tries something or a situation starts to develop extinguish it as quickly as possible. This is a time when procrastination is not an option. If she is critical thank her for her opinion you will think about it. If she is rude thensay we try not to talk like that to one another as it sets a bad example for the children.
Before your MIL comes sit with your husband and discuss the ground rules and pitfallsand agree a strategy. She will try to make trouble so make sure that you are united whatever she tries. If she detects friction in your relationship she will exploit it. Make sure you know when the visit is ending it is easier to endure when there is a finite time scale. And resolve the situation with his brother, how old is he and what is he doing to make his own way? Start making plans for his brother to be independent, living with you is not a long term solution for him. Is he planning to go to college? Does he have a job? what are his plans? If he has none then get him to make them and give him a timescale if necessary.
Get counselling from a professional, a third party will be able to make your husband hear what you are trying to say and they will also help you to understand what is going on in his head too. His lack of personal care may be rooted in other issues not just in laziness. They will also help you to stop abusing one another. This is a time when you must think of the children and a home where parents treat each other badly is unhealthy it has to stop and you must do what ever it takes to stop it.
You have taken on too much responsibility and quite frankly something will give. You need to create calm and peace in order to understand how you feel and to be able to address the issues in your marriage. Clear the clutter in your life.