Background: I've been with my DP for three years, living together for six months. He has two DC 4 and 7 and I have one who is 6. Everyone gets on really well. His custody arrangement with the girls is that he has them every Saturday morning until Sunday teatime and one night mid week. This isn't a court order just their arrangement which has been since youngest was born as they split when his ex was pregnant. It's his 7 year olds birthday soon which is on a Saturday and his ex is planning a party at her house for DC's friends. My DC is invited. We'd arranged to pick up the kids after the party and then back to ours. On the Sunday my sister is hosting family to celebrate my grandma's 100th birthday and I've got family coming from America whom I haven't seen since pre COVID. We'd arranged it for the Sunday to work around 7 year old's party so the girls can meet many of my family.
My DP received a text from ex this morning asking what we are doing on the Sunday after DC's birthday. I don't know this level of detail about their life nor do I think I should and it was clear it was leading somewhere. He then told me when soon to be 8 DC was over this week she'd asked what we were doing and didn't like the sound of my family gathering as it's her birthday weekend. So he texted back that we're going to my sisters for family party. Ex said she's going to keep the girls and they'll carry on the celebrations at hers as DC 'always misses out'. AIBU to think that my DP should be standing up to her and insist or AIBU for wanting the kids to be there at my family's gathering? I've very sadly lost both my parents and apart from my sister don't get to see wider family very often as we're dotted about. I feel like this is such a big event I'd like all the girls to be a part of it. My other sister who lives miles away can't make the Sunday either but can do Saturday so I feel like letting family know we can now do the Saturday, My DC won't be able to attend the eldest DC's birthday party but she will get to see her auntie and cousin which is a fairly new relationship (long story) and one I'd like to kindle as we don't have a big family. What would you do?