I totally understand where you are coming from. I have a crazy sister in law too, but the reasons she drives me crazy are a little different than yours. Mine is just under 30, she has 5 kids by 5 different guys, and 3 of her kids are in foster care. She is a compulsive liar, you know, lying comes as easy as breathing for her...If your sister in law has cheated on her husband, chances are she will keep doing it, and if she cares at all for her child, she will do what it takes to make things right. You can not blame yourself for what she does, nor can you allow blame to fall on you for her actions. The best thing you can do, and trust me I have had to do it, is show love, but tough love. Make sure that family knows you care, but do not tolerate unacceptable behavior, if you see it coming, walk away. It is difficult, but it has to be done. I have had to do it with my sister in law and my mother in law. They think that we are supposed to pick up the pieces where they screw up and it's just not gonna happen...we have our own family to take care of. If you need to talk, feel free to talk to me, I know it can be very comforting to talk to someone who has some idea of what you are going through!