Why would my brother who is advanced refuse to come gym with his reasonably attractive younger sister?

[Anonymous]

[FilmGuy93]

He's either worried about getting a throbbing erection watching his sister on the squat rack, or he views the gym as his "me time" and doesn't want that time encroached on.

[OlderAndWiser]

Because girls who don't already know him will assume that you are his girlfriend and they will stay away from him. Maybe that's the explanation.