Help

Divorce her. Cheating is the ultimate stab in the back.

Looks to me like you're already doing it.

Backstabbing can not be mutual and simultaneous, and they both cheated, seemingly at the first opportunity. Make that seemingly, definitely for the poster.

This would be much easier if you didn't cheat on your wife and continue to do so. Cause, two wrongs do not make a right and it never will. It only makes a big wrong. I understand you were thinking about the kids but you did it the wrong way. As most have said, You should have went into the room when she was in the act. But now, because you didn't and you were to confront her about it. She could be that type of person to deny it until you have definite proof..and then two, Cheating on her back especially with her sister to the point she's pregnant..you're children are going to be confused about their new Brother(or Sister)/Cousin. Now, if you were to confront her about it and try to say anything righteous and stuff the fact that you did the same thing will just make you a big hypocrite..But, basically all you can do that will perhaps cause the least amount of damage is Divorce her, Take what's yours. And live with the sister taking care of the baby that still will be confused sibling wise..And, also..If she was still cheating and continuing to do so..and you did the same instead of breaking it off right there..That's just wow. That kind of makes it seem like you're okay with the whole thing so long as you have some person you love. Apologizes if you take this the wrong way but that's how I see it.