Here's the thing. We have a nuclear family tradition (parents, 2 DCs) of going out together on birthdays. We have a small family. My parents live abroad. My sibling dead. DH's mum died almost 30 years ago. His dad is not engaged and on wife 3. It is DH's BIG birthday. DD is student as far away in the UK as it is possible to be from us and I have secretly arranged for her to fly back, get a train and meet us in a lovely restaurant to surprise him. It is a big deal. We do not do this often. Meanwhile, DS has lost his 4th job this year (really) and from two days ago currently home with us. This evening told me he has invited current girlfriend to come on DH's birthday evening. They have been together for 6 months. I hit the roof. AIBU