I would like to ask why the mother has so much vitriol, I don't believe for a moment 100% of what he is telling you
Sites like this must be full of so called crazy, abusive, narcissistic exes because that is what you are led to believe if you are only fed one side to the story.
I would always suggest digging to find out a bit more about the mother because for all you know, she might have been extremely badly treated by her ex ( your partner) which is what caused her vitriol in the first place.
A quick perusal on here would suggest husbands who don't pull their weight at home, have an affair, lack of sex, too much alcohol/ drug/sport/ selfish/ porn / gambling habit / abusive etc to be a cause of anger and upset in families where the wife is left to cope with an unsupportive husband.
Is your DH perfect? Any red flags?
Is he the best dad he can be?
His DDS are the product of their environment and the role models in their lives.
If they are as rude as you say they are, do you honestly believe there is no reason for that?
You are very naiive to think your partner hasn't had a big role in that.
I hope your dd hasn't had to deal with any fallout from all this dysfunction your relationship has encased you in.
If I was your dd I would be appalled at how badly you are treated and I would want the best for you.
Please have higher standards for yourself op! Partnerships never exist in a vacuum.