Serious sister problems (long)

I am so sad. My sister has been having a really hard time with her husband for the past year,(since my nephew was born) not to mention her eating disorder and PPD. I tried to give my best advice to both her and my BIL (who is no angel but has grown on me and is now my family). there was a rumor going around a few months ago that she was cheating on my BIL with her boss and i confronted her about it, and she said that it was a rumor and told me how it came about, so i kind of filed and forgot about it. To make a long story short, this past weekend by BIL found out that the rumor was true, she has infact been having some sort of a realationship with one of her bosses. Who not to mention, is engaged. My heart is breaking for my BIL, and i am so mad at my sister. She hasnt talked to me about any of this, she has been confiding in one of our close but brainless friends, which makes me sad too. And another thing is this jerk of a boss's fiance has no idea what a creep this guy is. I feel like hunting her down to tell her so that she doesnt marry this guy. and the scary thing is, this is all local, this woman could be on this very site.
I cant even look at my sister, and everytime i talk to my brother in law i cry, im think i am more upset about this than my sister.
what do i do?

Wow this is a seriously tough situation. I think the best thing is to not get involved and let them work it out. Here are some hugs

The hardest thing of all... nothing. Just sit back and be there for them. As awful as that sounds, this isn't your relationship to fix. Just make yourself available, to both of them, whenever they need you.

The only thing you can do is often to listen and try not to give adivce-which is definitely difficult. This is something your sis and bil need to sort out as a family . Good luck with everything and I hope it all works out!