Find him a girlfriend. Or a friend. Be nice to him, he's just a little lonely and wants a friend.

Three way?

sorry that was really gross, but i had to say it.

I Think it's sweet. He's probably not that old.

Voyeuring, Incestuous, cuckold?

this is either sad, weird, or understandable. if he is a lot younger, it makes sense if he just wants to hang out with you. if he is around your age, its just kinda weird unless he is just really lonely. but if he is around your age or older, its understandable because i myself have a younger sister and i feel sorry for the first boy (and every single one after that) who knocks on our front door asking for her. so it makes sense for him to be protective

My older sister used to bring me out with them (she and her now-husband) sometimes. Just to movies and stuff like that but there wasn't any harm in it because they had been dating for like a year anyway (not to mention I never asked to tag along)... I tried to be nice to my little sister in the same way when my first bf and I were dating but that didn't go over well since there's a smaller age gap between us...

That's not weird at all no matter what age. If he got along well with you and your bf I say you should let him hang out with you but if he's younger try keeping it PG. My bf and myself always hangs out with his older brothers and we get along great.

You should've specified how old you brother is.

In some cultures it's still practiced this way. It's called chaperones.