What would you do if a family member made a pass at you?

No I don't think it's a good idea to tell your sister anything, at least not till after the wedding. To be honest I'm not sure whether or not you should tell your sister at all. This may really affect her marriage and you'd feel very guilty. On the other hand by not telling her you might be preventing her from realizing what a low life her husband is and that he may cheat on her in the future. Tough situation to be in. Like I said for now I wouldn't do anything and just focus on your wedding. Later on after you settle down hopefully you will be able to make a decision. Good luck to you.