I agree with your family on this one, you do need to put a period on this R and move on. The R is over, he's with another woman. If he decides to change that, then you can work on another R with him, if that's what you want.
His R with his kids is up to him. You should do nothing to make it worse by bad-mouthing, etc but it's not your responsibility to make it better.
Me 57/H 58 M36 S 2.5yrs R 12/13
Let me give up the need to know why things happen as they do. I will never know and constant wondering is constant suffering. Caroline Myss