I'm a little unclear, at one point you said they're related by marriage? If your bf is related, then you need to work on sorting things out. He should either help you, or perhaps you can go to the niece directly & have a one-on-one conversation & really clear the air.
If they're not related (by marriage or any other means) then I don't understand how he would place them above you for holidays, and that would be a red flag imo.
All that being said-I do consider myself very friendly with one ex and his family. But the circumstances are that his mother & father were good friends of my father's when he was a young man...the two families were friends for years and I saw no reason to stop that just because our relationship didn't work out. I was 14-16 then too, and I'm 31 now. I'm not going to stay mad about something from so long ago

My dh doesn't feel threatened by it either-he even helped my ex work on the mother's car once.
So, there are situations/circumstances where ex's can remain totally platonic friends. Every situation is different.