Most people suck, but guys who flirt with your sister as some kind of head game?
well, that's just tacky, and if it were one of us 3, (I have two sisters) we'd send the boy home to his momma with his tail between his legs for some home training.
We always clarify first, if the flirt is subtle. Don't wanna be over reactive or insecure about a 3 second pat on the shoulder. But if it's deliberate and obvious? He ain't worth being had by one let alone the force of two or three. No man boy can handle being a toy between sisters without somebody feeling a bit used.
Discard him or disregard him as being responsible for his actions. Either way, you gotta live with your decision.
somebody's out there that will know you're special apart from any other and he won't ever play games with you about it.