John, Maybe I didn't put it right. I wouldn't feel that her actions could be justified, so as a parent that would make me even less likely to be accepting.
Hopefully I will never be in that position. My SIL is a pretty decent guy. He also has a retired minister for a father. If he ever cheated on D he would probably have to answer to them before me anyway.
Anyway, it sounds like your parents are coming around.
I imagine that sometime in the future you may get more of an appology. It's just less painful for her to blame you right now.