Boyfriend said that she was his sister?

He followed a girl on tik Tok recently and he says most girls are his sisters on his friends list. So I did ask is this girl your sister? He said that she is when I know for a fact that she isn’t cause I found romantic messages between them in his photos from years ago now he’s followed her again recently. Should I be worried about this behaviour
Boyfriend said that she was his sister?
Post Opinion