New GF needs to be out of the picture. It's absolutely none of her business, and she especially has some audacity going to your parents to smear you.
No matter what is going on between you and your ex, his family should want to see the kids. The fact that they're alienating their grandchildren to try and hurt you shows that they aren't mature enough to be caregivers to young kids in the first place. They have the chance to be a loving, safe, nurturing place for those babies regardless of all other drama, and they're throwing it away. Shame on them. They're missing out.
Do everything you can to keep your ex away from both you and those babies. This isn't me being an alarmist. He has proven that he's capable of harming all of you. Document every single time he steps out of line. Get an emergency custody order, protective/restraining order, anything and everything you can do.
I'm so sorry you're going through this. I'm trying not to get mad for you, but I'm the mother of a little one and it's just fucking unacceptable. May you all heal from what you've been through, and please stay safe.