Am I overreacting? I feel like something like this happens more often than not…

My SS’ bday was this past week. I suggested my DH let him skip school for the day and take him snowboarding (cheaper tickets on weekdays) for the first time since he’s been begging to go for over a year now. I helped both DH and BM find him gear the night before (since they both procrastinate like no other and I do too but daaaaamn it’s a whole other level). We all decided the gift would be from all of us (me, DH, BM) since we’re all tight on money and the outing was not cheap. So my DH takes him and was saying my MIL baked a cake and wanted them to stop over after. I was annoyed since she always does this with their bdays and makes us carve out time to go to her place for a cake no one asked her to bake (we ended up with 3 different cakes for my SD’s bday and had to plan the day around getting it from MIL). He asked her if we could postpone til the next day but she insisted saying she also made food and the water was already boiling (major eye roll). So he called me to tell me that they were going and he’d call me to sing happy birthday over the phone. He calls me when they get there and I speak to my SD over the phone and find out BM is there too. This made me extremely upset to find out that while I’m busy working, they have no problem having BM’s family moment without me. My MIL texted me later that she was sorry I wasn’t there but food was ready!! Kissy emoji face. I was upset that my DH knew she’d be there and didn’t say anything and that clearly my MIL had texted BM inviting her over/letting her know the plan but never sent me one fricken text about it. So right now, I’m furious with my husband and feeling utterly betrayed my both him and my MIL that they could sit there and think it was okay to do that without me with the golden excuse “it’s for the kids”. I understand it’s SS’ bday and was for him, of course I want him to have a great bday shit I’m the one that planned it! But it’s eating me up that I was left out and no one seems to care or think there’s anything wrong with it…am I overreacting?