personally, i think you should stay out of this and try to convince your stepmother to do the same. while you might understand your step-sister's perception of her marriage, no one ever really understands what goes on in someone else's marriage except *maybe* the two people involved.

ok, answering your stepmother's questions wasn't the best move, but she *did* ask and should have been ready to accept whatever answer she was given. but that's not really gonna win you any points with her right now. what might work is explaining that you shouldn't have answered her question because your stepbrother's marriage is none of your business and you want to stay out of it and let them try to work it out or not. you might also explain that you actually don't think your stepbrother is a bad guy and that it's almost impossible for one person to destroy a marriage or make one work, so clearly both your stepbrother and your sil likely contributed something to the current state of affairs. laying blame isn't going to help them resolve any of the problems and even if they divorce, they're still going to have to find a way to get along because they have a child together.

i don't know your relationship to your stepmother, but it might help for you to tell her how it felt for you to hear them talk trash about your ex--that it really didn't help you deal with anything. she probably does think she was being supportive by doing that.

ideally, your stepmother should see a counsellor to deal with her feelings on this so that they don't interfere with her dealings with her son and possibly future ex daughter in law and everyone else. her son's divroce is not about *her* and it's really not going to help him or anyone else to have to deal with her problems with it in addition to their own.