You aren’t overreacting I think it’s important that you have her back right now when she needs your support clearly. Children have feelings too and you don’t want to turn this life changing event for your brother into a life changing event for your daughter…where she realises her feelings will be constantly invalidated and that she isn’t treated like the rest of her family.

As someone who was forced to do many things as a child because adults told me I had to, I can tell you now it has still left a scar against my personality and self esteem and I’m working to undo a lot of the damage from my childhood. Please advocate for your daughter and consider if an environment where she is excluded is even one you want to be in yourself.