I guess I’m the dissenter here. I too have an unusual situation in that I actually live w my ex as friends. We bought a house as an investment. We were apart during much of my sons childhood. We had joint custody and are both very mild mannered people. But we didn’t do things in a totally combined group. Jealousy , eg territoriality is a normal emotion. She is nice to the kids, but I think forcing her to associate unnecessarily and against her will with your ex goes against human nature. Of course she needs to know that you are important as a couple.
I understand the new girlfriends position. You could have two smaller get togethers for big holidays rather than trying to squash everyone in together. She should not have to feel like an add on in those get togethers.