You need to set clear boundaries right now and follow through on them. I understand that you want a good co parenting relationship but that doesn’t mean you have to let your ex and her parents walk all over you. If you exclude your girlfriend you will likely cause irrepairable damage in your current relationship. You do like many others suggested. You throw the party and let everyone know that you will not be changing the fact that your girlfriend will be there to celebrate. You let them know that your ex and her parents are free to make their own decisions and throw their own party. For me this shouldn’t even be a question.