What to do about my sisters bf and his rudness towards me, when i live in thier home..

I am currently staying with my sis, her two teens, and her boyfriend. I have problems with anyone in the house , except when my sisters boyfriend (im gonna refer to him as J) starts making rude comments towards my sister and now myself (which i could care less what he says about me, but it really bothers my sister and stresses her out), for no reason. I dont say anything to him, or even let anyone know it bothers me bc i feel its not my place to say anything ever bc its thier home and relationship. But hes going way beyond too far every chance he gets, and its really effecting my sister and how she feels about herself as a mother, a gf, just a woman period, and not to mention its WRONG! Ive noticed that he takes my niece and nephew out for ice cream at times too, and when they get back, the three of them start talking crap to each other about my sister out loud trying to make her mad, like its all a game. Laughing and cutting up untill she ends up losing her cool and yells at everyone. Then the kids get mad at her and the disrespect gets even worse towards her. And her bf does nothing but sit there with a smile on his face or he pays no attention to anything but his phone. She always ends up in tears and drinks herself to sleep. This happens oretty much every day . I try to talk to her about it and shes aware of what is going on, but she doesnt know what to do and puts up with it, bc as long as her kids are raken care of and have everything they need, she feels shes comfortable with the way her life is right now. But its so obvious shes not. I love my sis and my niece and nephew and i want to help them without over stepping any boundaries. Am i wrong for that? Should i just mind my own business? Or is there anything i can do to help her and the kids at all? I hate seeing this happening to my family. What should i do?