I lost my sister 6 weeks before her 26th birthday. We didnt celebrate birthdays or anything for the entire year. It was difficult for all of us. It still is. After that we started again. Mourning is very difficult. But that doesn't mean we stop celebrating the lives we are surrounded by. Honour the memory of your brother. But also honour the birth of your daughter. May your brother watch over her.

It is okay for the adults to stop celebrating, but dont pass the burder to the next generation. Death comes for everyone. It is hard to deal with. Therapy helped me a lot to unpack all the emotions, I would suggest the same for your mom and dad. It sounds like they are still struggling, I dont blame them it is very very difficult.

Soft YTA because you & your parents need to find a better way to grieve that honours your brother as well as celebrates the life of your daughter.