Sister

That one one person that if you're too close too, or spoil too much, some people out there start thinking you're into incest.

Boy #1: My sis said she's been wanting some chocolate recently, guess I'll surprise her.
Boy #2: Your sister?
Boy #1: Yeah, why...?
Boy #2: You too are always so close, is there some incest thing going on with you two?
Boy #1: You watch too much porn and anime bro...