Similar to the Brother Zone, except to a woman. Pretty much, the guy/girl you like comes out with: "I like/love you like a sister". This can be done by men or women.
Chick 1: Hey sweetie, how did it go with Dan?
Chick 2: Shit. I just finished pouring out my soul to him when he tells me that he loves me, but only like a sister.
Chick 1: Shit. Sister Zone sucks.