Mean mother and mean brother.

Hi,
I am new. My parents passed on when I was a boy and raised by my aunt and uncle who are now in their 80's with failing health. They have two biological children and I had one bio brother who was killed. I never caught on until after a while that when my brother was alive he was the family scape goat and his behavior did not help any. After his passing, it seemed obvious that this role was passed to me. My aunt more so but my uncle also found falt with me - a lot of it and it was either yelling at me or the silent treatment. I did learn about boundaries and actually work as a MA mental health therapist. When I did set boundaries such as : "I don't allow ppl to talk to me that way and I would leave the room." I made some clear errors in the boundary setting and honestly it was out of fear that I did not directly handle it in the proper manner. I was afraid they would die soon after and I would be left with remorse. Although, I mistakenly landed into a few shouting fights with my aunt and uncle and their one son, I was tap dancing around at times - many times- to avoid confrontation. Here is my challenge: 1bother lives with them is primary care taker by default since he has been living with them for over 12 years. when my aunt and uncle were healthy, they made his bed, payed all the house bills, washed his clothes and every other need including using my dads car whenever he chose. They enabled him. He is on disability for health issue.
He is acting like a marter. He is holding a ton of resentment toward his brother and I for not being there more for my aunt and uncle. He is use to doing what he wants, when he wants. Him and I were in two massive arguments that got into name calling, profanity and almost physical. I learned from that lesson as I was ashamed of my behavior. I have not argued with him since. However, he has become more bitter and more bossy. It appears he will not saying anything about his resentment to his older brother but is arrogant and mean when I am there. My aunt also gets angry and now it has spilled over into being angry at her care taking son as well as many others. When I go to the home it is full of sickness (which I can handle because I do love them- but it is all the bitterness that my aunt and her son have which spills onto me that I can't take. To make matters worst, my uncle who was very sick has started to think more clear due to a new medication and I saw this past christmas week that he was in a very slight way becoming critical of me. To add, now my aunt's son got help which is a lady who comes to her house 7 days a week 8 hours a day. However in order to get this help, they needed to have no real money. my uncle never told the service that he had 35000 dollars in the bank and when they found out, he had to forfeit the money which messed him up emotionally. He is the type of person where money is everything.
I will stop all the details. 3days ago, I left the house in the middle of the night with no warning. I snapped. I have not spoke to them since. I was thinking of writing them a letter expressing my love and gratitude and let the mother and son know I want to and will help more than I have been. I live in another state. I want my aunt and uncle know I will be there for them but only if the yelling and fighting stop. I do not want to be around their son at the moment because of his actions -i.e waking me up in the morning by using profanity because something did not go his way- I sleep on the sofa when there so all he had to do was walk by me and raise his voice which is what he did. Should I call and confront my aunt and cousin which with my cousin will more than likely only to him yelling and cursing and probabally with my aunt also. I thought a kind letter was the best route and then follow up with a call. I thought they would feel the least defensive and at least read my letter. I could call both of them and tell them the BS stops now if you want my help because I refuse to be around this toxic environment. Make it short and sweet. Obviously they are not concerned about my feelings to act toward me as they do and yet I am being or trying to be considerate of their feelings.
Every time I leave that house I am emotionally sick for several days.
other factors- on thanksgiving my aunt got into a blow up with her granddaughter, son and another family member.
the second son has a wife a she is now becoming mean and got into a fight with the care taking son on thanksgiving. it was a family explosion at dinner.
christmas she was at the house and was raising her voice at my uncle.
this is a sick environment and although i want to honor my aunt and uncle for raising me, it does not give them and their son a license to be mean to me.
I hope I expressed this clearly, but please ask any questions if you are confused about this. I just want to add please keep in mind that this is not an age related problem with my aunt she has been "abusive" to me in past.