n. like the friend zone but for girls; the guy cares about the girl but due to whatever reason (small in stature, young looking, one of the guys etc) he sees her more as a little sister than someone he would go out with; tough for the girl to ever get past this stage; they might think you're kinda cute but that's as far as it gets
"Wow he's cute! You should get on that!
Nah I'm in the little sister zone
"He's super sweet but whenever I try to make a move, I just get stuck in the little sister zone