This is an ugly mess that you're sister shouldn't have expected you to cover her decisions to cheat. Now that you've told her husband about that other lover I think it’s best for you to take a step back and allow them to deal with their marriage. You're not responsible for your sister or her husband. You don't need to be there and have them vent and cry to you. It’s a lot of stress no one needs in their life especially when you're not involved.