Excessive love for one's sister(s), often past the point of ordinary brother-sister/sister-sister relation. Often used as a tease/insult for a person who's overly protective of his/her sister, and is shortened as sis con or sis-con.
Derived from Japanese word "shisukon" (sis con), and is often used in manga and light novels.
M: Ota has a sister complex.
O: like hell I have!
M: Please, you've got your sister's picture posted all over your room, your facebook profile picture (and several hundred picture) are you and your sister, and you get pissed whenever we talk about your sister.