It's horrible but you have to make a decision as to how you want to proceed. You are fully aware of the situation and have come back into the relationship knowing exactly what has happened.
If you really dopn't like it, then I would say that because of whats happened you are not happy with him spending time with her alone (not seeing her at all is impossible as he is the kids uncle). And if he's not happy to do that, then you walk away.
I would point out that if you "hate her for what she did", it tales two to tango so what are your views of your bf on this?
Also, without wanting to unnecessarily put the cat amongst the pigeons, do you not think her husband/bf deserves to know?