As someone who lost a child in a similar way, I’d still say (a very light) YTA. You still have to be a parent to the children you have. It may be hard but you could’ve at least gone to the ceremony and then gone straight home. I lost a child and also before I was born my parents lost twins and the day they lost the twins coincidentally ended up being my birthday. My parents never made me feel like they can’t celebrate my birthday because it was the day they lost the twins. I really feel for you guys but you missed out on a huge day for your still living child and I can see why she would be very upset, especially since you have to book weddings very far in advance so when she booked this she might’ve not known that it was a hard day for you guys. Nevertheless, you need to be there for your children. If I were her I would find it hard to forgive you for missing my big day. I hope you guys can work this out and I wish you nothing but the best when it comes to mourning your child because trust me I know how it feels.