The children and the sister feel the way they do, you can't change that, you can only change how you feel about it.
I've not spoken to my father for years, he left for another woman. He then left her and is with someone else and has a child with her.
Whilst I have no issue with the 2nd woman he is with, I also have no desire or urge to be in contact with my 1/2 sister, or her mother, it would mean having contact with someone whose actions and attitude were abhorrent. (my father)
Perhaps when she is older and if she wants contact with me, we will have a relationship, but I really feel no attachment or need to know her. That's without having an axe to grind with her mother.
Does that make me horrible? I don't know, I can't help how I feel. As it is the issue has never come up anyway, as I am not in contact with my father, though my brother is.
And as for forgiving my father and woman he left my mother for, I don't think I will.
I'm generally a happy go lucky person, but I would not choose to have people who behaved as they did as friends, and my feelings won't change just because my father is "family".
My feeling is that if he was so "unhappy" with my mother he should have ended the relationship before starting another....what he did was awful and unforgiveable, just because it happens a lot does not make it ok.
So again, really you need to accept that they feel the way the do, and move on for your daughters sake.