Death is so weird. Like should she just forget about her birthday for the rest of time?

Alright, this might sound insensitive to some. But you have to move on. I realize that one year isn't very much time when it comes to grieving the loss of a loved one. But throwing a hissy fit over the Aunt celebrating her birthday seems like an overreaction and isn't fair in the slightest.

The aunt respectfully visited with her grieving sister, spent the day with her, then went to work. The fact that this was also a surprise birthday celebration at the end of the night makes the blowback even more absurd. This was an intimate birthday event, shared between two people. It's not even about disrespecting the nephew who passed away. At this point it's about having respect for the sister who STILL has a birthday.

Sorry not sorry, the OP is not the a**hole.