How to deal with a family dead naming one of my kids

I have 3 kids. Two daughters and one kid that is gender fluid and goes by he/they pronouns. My gender fluid child is 16 and they actually didn't change their name because of his gender identity. It's because of his dad. He goes by Sky but his dead name is Christopher (Chris) Jr. My kid lived with his dad while I was in prison and his dad abused him so he didn't want to be a junior anymore and had been going by Sky for 2 years. We see my family every holiday and we are going this weekend but they have a bad habit of dead naming my kid and it triggers them badly. They're constantly calling him Chris or CJ. They don't care. They don't understand it causes them a lot of distress because their dad was awful to them. But they don't respect the name change.