Is it weird to constantly refer to your sibling not by name?

I have a pet peeve which probably sounds dumb. But anyways my boyfriend constantly refers to his sisters as “my sister” instead of their names so I NEVER know which one he is talking about and it gets annoying. When he says “my sister did this” or “ask my sister blah blah” etc. I know them well so I don’t know why he never says their name. They do the same thing and constantly say “my brother” but it’s less annoying because he’s their only brother. Wouldn’t it be weird if I constantly talked to them and said “my boyfriend”??

I think I know why but it still doesn’t make sense. He calls them bong and bong thook in Cambodian which is just a nick name of respect for older siblings and people. They call him ah oun which is a guy younger. In thai, Chinese, and other asian cultures we have our own nick names for older siblings but everyone I know including me and my family still refer to our family to another person by their name. Only time it’s “my ____” is when someone doesn’t really know them.

Its different with “my mom” or “my dad” lol I mean he refers to the rest of his family by name except his sisters.

P.s even when talking to their aunt, cousins, etc they still say “my”