I (39 F) have been with my bf (36 M) for 5yrs. We live together with my son 

(18 M) in a small two bedroom house that we are currently renting. For the last 2 1/2 years, I have been our sole income due to my boyfriend's health issues. This has put a great deal of stress on me, causing a decline in my mental health,as well as put a tremendous strain on our relationship.
Last week, my boyfriend's sister reached out and shared that her boyfriend has been abusing her and included a picture of the black eye that he gave her. My boyfriend insisted that she come stay with us, as she has no friends and he is the only family for 500 miles. I told him that I understand why he wants to help out his sister and having been in her shoes in the past, I want nothing more than to get her safely out of her current situation. I explained to him that we can't possibly afford to house another person on my income alone. His sister is not employed and wouldn't be able to help with bills or food. There's nowhere for her to sleep, as I sleep in the living room due to my boyfriend's insomnia and his sister has a drinking problem, which would cause drama in the household. He got angry and told me that I'm "being selfish". I told him we could try to find a safe place for her to go so she can get help with her drinking, find housing and a job but he wasn't interested in hearing it. He packed some of his things and has left to stay with a friend because he doesn't feel that he can trust me anymore.

So AITA??