OP..
Listen man..
Learn to stay out of other people's business! That include siblings, homies, etc..
Next, your sister is foul AF! Her ex BF lost a sister and instead of giving him moral support, she cussed him out & broke up with him over an assumption of cheating. Your sister had not one no iota of proof. Didn't give her ex BF the benefit of the doubt that the ex BF was going through some emotional trauma & grief and you and your sisters wanna know about his character? Crazy!

OP, tell your sister that Commish said for her to grow up! She was wrong! She didn't trust her ex, never did and never will! Tell her that it was uncalled for for her to disrespect him and give him problems while he was grieving the loss of a sibling. She should have went with him to the fuberal, if she was concerned about her ex stepping out on her.
All this other crap about if the tables were turned bs doesn't matter now. They broke up, right? My thing is for you to help your sister mature and work on her anger management skills. She isn't ready to be in a relationship and I hope dude don't get back with your sister!
Just giving you my perspective..