Aita For Asking My SIL If She Really Wanted An Affair Baby's Child After She Asked My Wife To Be 

Nta. Op was a boss there.

"My wife was laughing, and she hasn't for a while" That there tells me that you did the right thing. The pain her family put her through still hurts, and what your SIL did was say "now you can be a useful tool"

Like barely the AH the family was all fine tormenting her as a little girl now it's "Don't bring up the past" when they want something out of her and all OP did was "ruin" that for them.

Lol not only NTA but also OP is 1 of Reddit's up most W Husband's, and as for the Sil ifs thats how u treat your innocent little sister maybe you don't deserve kids

Well, I laughed, too, just now. Still smiling.

Yes the AH but for the good reasons.

They treated her like trash and then asked her to do something like this, NTA, tho you should have sounded more like you were defending her.

Nope, also if she actually agreed that would be grounds for a lightning speed divorce.

Justified AH. Sure, the comment was a low blow joke but it had the results OP wanted. Made his wife laugh, called out the SIL, and exposed the hypocrisy of the in-laws. Bet you if she had accepted, they wouldn't have paid for anything, leaving the whole burden of the pregnancy on OP and his wife. I have no proof but neither doubts.

Yes YTA your wife was already on the fence about a 3rd baby and did it anyway to expand your family with love. You should have defended her by saying "i don't think that would be a good idea medically for my wife." The fact that she was laughing at it so much sounds more to me like a nervous response and not knowing how to respond without causing an issue.