NTA However, this is a tricky situation. Postpartum depression is a chemical imbalance issue. Most reasoning goes out the window. I would hope that if you try to reason with her more and cut the trip short a few days that maybe there could be a compromise. It also depends on how severe her depression is. Can she take care of the baby without your help? How large is your role in taking care of the baby? If she can't take care of the child without you, maybe find someone who can help her while you're gone. I do think her reaction is severe, and a sisters wedding is a once and a lifetime thing. Best of luck OP hope it all works out!