Wow.

So obviously you’re not in the wrong for wanting to divorce him and in fact I would say that you are a butt face to yourself if you stay with somebody who does this. Also I know that this is going to just destroy your son but he needs to know about this. There is a nonzero chance that if she is banging your husband that she’s also banging other men and your son needs to know that she is not safe to sleep with. Also yeah this is going to be hard for the granddaughter as well, unfortunately nothing about this is going to be easy for anybody. Also when she dumps your husband and she will dump him he will try and come back and you do need to say no.