About deadnaming

So some context my cousin is trans (ftm) and is closeted to everyone except me (I think? idk probably also to some of his friends)

So whenever we hangout at my house it feels really wrong having to deadname him Infront of my family, because dude is one of my favourite people and it feels really awful to have to deadname Infront of family, I asked one time about deadnaming Infront of family and he said "yea ofc do it" but it still feels rly wrong

Tldr: my cousin says it's fine to deadname him Infront of family but it still feels wrong