what would you do if this was your sister?

this isnt a "give me advise on how to break them up" post. Its a "how would u handle al the inapropriate comments, and situation" post.

ok so im going to make this back story as short as I can....

So in this case its my sister in law, but i have known her for so long that she is my sister, she has been seeing this guy sense like July 2015... He was married and has a kid.... I guess him and the wife were having issues, but the guy last summer had told my sister that their thing needed to stop, he wanted to work things out with his wife.. that lasted 2 weeks and she was back to blowing up his phone and hanging out at the golfcorse with him, telling him that she was going to report the wife (who apparently was emotionally abusive, wasnt in the house so i cant say nah or yay) and she could treat him better and all this other stuff... anyways he ended up telling his wife about the affair... he moved out she immediately moved in with him. Hes been divorced for 3 months and already proposed...

OKAY so she IS 21 and she CAN do what she wants, shes an adult blah blah blah. Where my frustration lies is that my dad hates the situation, my husband hates the situation, i hate the situation. We dont want to hang around this guy because

1. hes SOOO co-dependant.

2. he makes inappropriate comments about how he spent 3,000$ on her ring and that i deserve a better man because my husband didn't spend that much on my ring (nevermind the fact that I helped pick out the ring and it was the ring I wanted). and comments that HE can support my sister and how she will never need to work, and he feels bad for my mother and I because we do have to work.

3. My sister doesn't seem to get that she, ya know, helped brake up a family. and is trying to come into his kids life and raise him the way she thinks he needs to be raised

theres just so many reasons why we dont wanna be around them. But when we dont she throws a fit and my husband and I get chewed out from mom because we "need to be supportive". we "need to rmake him feel accepted in the family. hes insecure and we need to help him with that."

we are so busy tiptoeing around this guys feelings all because my sister said she was going to pick this guy over her own family and mom is afraid to loose her... which i think is bull crap.

my husband and I got kicked out of our room on vacation that we helped pay for all because the fiance and his kid were coming. I just dont understand how someone can walk around so proud of themselves when they have done something so wrong, and then get mad at those who arent supportive. and like what do I do if she asks me to be in the wedding?

Im at that point where i want to say something because shes always like "why are you upset?" "why dont you wanna hang out anymore?" but we dont wanna say something because itll start a huge fight. Like i want to be happy for her and im just not. And sorry if this whole thing makes me seem like a self centered B****. just feel like we are at an intersection that is a death trap no matter which way you go