It was assumed my brother and I had a relationship fairly often in high school just because we arrived at school together and left together. We didn't interact other than that really except when I had to break a ten for our lunch money and hand him his portion. Idk why people didn't realize we were related, especially since we share a last name but whatever it was usually cleared up pretty quickly.

The most annoying time though was when a girl in my first period class noticed he and I always showed up to and left school together (because ya know we were going to the same house) so she took it upon herself to write my new boyfriend a note telling him I was cheating on him. He confronted me and I laughed it off saying she was reading too much into it, that was brother. My new boyfriend apparently didn't know I had siblings though, so I found my brother in his lunch and yelled across the cafeteria to ask what mom was working that week. It helped clear up quite a few people's confusion, I had friends that I'd known for years telling me they assumed I was an only child until then.