I wasn't the crazy ex that you are describing, and I'm the one who left OH and certainly didn't want him back but I stayed very close friends with his parents. Like you, his new partner was very unhappy about it. She first told it to my ex, my ex told his parents and their response was that they were very happy to welcome her in the family and of course they would respect her position and not invite me at the same time than her if it was an issue, but that she had no right whatsoever to tell them who they could see or not. We'd grown very close whilst I was with their son, they cared for me as a person in addition to the mother of their grand children and considered me very much like another daughter.
Unfortunately, at a meal there, she told them herself that she wasn't happy about it and more or less told them off for it which really riled them off. This led to her refusing to see them, and my ex supporting her and so stop seeing them too.
I remain close to them, seeing them regularly and they welcome my new partner in their lives just as they were prepared to welcome my ex's new partner. They even came to our wedding!
Before he met her, I asked my ex if he was ok with me still seeing his parents and he said at the time that he didn't mind at all and indeed, we met there a few times for big events after we separated.
I think it's very sad that she felt she could command who they should see or not just to please her. They are lovely people and would definitely have welcome them in their family. They just didn't see why it would mean having to exclude me from their lives.
You've only been together for a short time and you probably still have a lot to find out about the dynamics of the family and his past. Just focus on making your family work with all the kids and let his parents lead the life they want to lead.