Is This Normal?

My nephew (husband's cousins child, not true nephew, but she and I are close) has been calling my son his little brother lately and I've been feeling some type of way about it. I can't really pinpoint what it is I'm feeling, it's almost like a jealousy or a defensiveness? He's not a bad kid and I really enjoy that he and my son get along really well for the most part. It's just weird to me that he calls him his little brother to people who aren't family members. He's never said it around family members but at the park to other kids and hes 'narrated' little videos saying my little brother this and that.

I've never experienced anyone having this quirk or issue before so I have no clue if it's normal or if it's a red flag for something else?

My nephew is 8, and has both autism and adhd. My son is almost 3.