it's a tough situation to be in, but you're right in that she is your sister. i would just talk to her about it (if she wants to), but this is between her and her husband. the thing is, who really knows what goes on behind closed doors? if she were asking you to lie for her, that's another story. but, it seems that you feel shitty she is doing this and you know about it. it sucks, but that's just life sometimes.