I've seen many an engaged sister do what you've described.

Still, I feel that you should tell your sister your concerns, especially if she was drunk (or something like it) when she's flirting with these guys.

Hanging on to, kissing, holding hands or what not...is nothing to sleeping with someone, but that's just my opinion.

It could be because she knows she'll be married soon and she's testing the waters or whatever reason. I don't agree with it, but at least let her know that you don't think what she's doing is right...and let her go from there. Try not to sound judgemental or mean about it, just let her know how you feel and assure her that you'll be there for her if she ever needs someone to talk to just in case she needs to let it all out.

After all, you are her roommate and sister.

Still, whatever you do, it's not your place to tell her fiance, no matter the outcome...no matter how close you are to him.

Good luck!