In A Really Big Pickle

I have a major dilemma and I don't know what to do.  My brother leaves on Tuesday morning and I feel like there is so much I need to say and I'm not sure if it's a good idea to say it or not.  There are three really bad things I found out so far during his visit:

1. My brother constantly cheats on my poor sister-in-law who is one of the nicest people I have ever met.
2. My brother is an alcoholic and has possibly started to mess around with his previous cocaine addiction from many many years ago.
3. My brother is a liar, always has been and apparently always will be.

I also found out that he's a bigger jackass jerk than I thought he was, but I've always known he was a jerk so I don't really think that counts as a discovery.

I really don't know what to do.  I feel like I should say something or slap him.  I know that if I try talking to him it's just going to turn really ugly and he's going to make me feel like crap just like he did earlier today when I tried talking to him.  I also feel guilty for knowing what my brother is doing and not saying something to my sister-in-law.  I know it's bad if I say something to her, but I feel horrible not telling her husband is a cheating alcoholic jerk who always lies.

This might keep me up all night thinking because tomorrow is probably my last chance to have a meaningful conversation with him before he leaves and I'm in a really big pickle.