Are we wrong for missing my nieces baptism

Technically only my son's are missing it. My son's who are 16 and 14 are going with their dad to an anime convention in June. They had been planning to go for a while. They've both worked to save money for their cosplay and my ex husband has been really struggling the last few years with cancer. He's in remissions and he promised if he survived he would take them because he's never been into anime, but he felt bad he didn't take more time to learn their interests. My sister called me up yesterday just saying she knows we aren't religious but it would mean the world if we could be there for her daughters baptism. I said sure and when she texted me the date I had to call back and say I could go but my boys are gonna be going out of state with their dad to an anime convention. She got irritated and said this is really important to her daughter. I said I would be there but her daughter wants everyone there. She said my ex husband is in remissions and they have plenty of years to go to a convention but only this time to see their niece get baptized. I didn't want to be mean but my boys don't care that much. I didn't tell her that but I said they already bought their cosplay and planned for the trip. My ex already got the hotel and everything ready. I'm not gonna make them miss this because she wants me to. My sister says "So you're gonna let her cousins miss out on an important moment of her life to play dress up?" I said this was already planned months in advance and I'm sorry. She told me whatever and that I shouldn't come and she'll pray for me then hung up. I feel bad but I don't want to take away something important to them. Especially since watching their dad fight cancer was horrible for them.