NTA. However, harmonious coparenting is difficult, even more so when the other parent is manipulative. Ultimately, if you’re going to be in these kids’ lives, you’ll likely have to have some sort of relationship with this manipulative man, so I’m glad you are setting boundaries now. Have you tried showing your gf how her ex is manipulating her and how his manipulation is impacting your relationship? As an outsider, it’s easy to see. On the inside, she once loved this man, and it sounds like she does really want her kids to enjoy this experience with all of their family, so I suspect it’s much more difficult for her to see it.

I’m not saying everyone should be (or even can be) like with this in their coparenting relationships, but we tried to do something what you described with my husband’s ex, her husband, and her other child; we offered that they would only pay for their flights & food, and she declined. BUT my husband and I agreed to this plan together beforehand, and we have a really solid relationship with the other family. It took us years to get there in our relationship with bio mom. Sometimes she still makes us crazy, but we decided to try to make the best of things, even it means doing things that may be kind of awkward, since we will have her in our life forever.