Telling family

Julie • Full time mommy to 2 amazing little boys. A 3 year old and a baby born March 16th 2016 🍀
So we live a few hours away from my brother and his wife. We are going to visit them tomorrow for my brothers birthday dinner. We haven't been able to tell them about the baby yet; I'm showing though so they will figure it out pretty quickly when they see me but I feel bad to announce it to them on my brothers birthday. Don't want to steal his thunder. 
We are all adults and I don't think my brother will care but his wife is very petty and I'm nervous of a negative reaction with her. 
Should I just pull them a side when we first get there and tell them quietly or should we have fun announcing to them? (Everyone else who will be there already knows)
For our parents we sent our two year old into the room wearing a "big brother" shirt. I'm just not sure about making a big deal at his birthday... It's awkward 
Any tips?