A twisted sister is a girl, typically in college and living in a dorm, who has "her boys" - a group of young college males who probably live on the same floor or in the same dorm as said TS.
She's the seemingly "cool", laid-back sister-type in this group of guys, but she's generally a boring bitch who's overprotective and maniupulative. She isn't romantically interested in any of these "boys," and they are not interested in her, but they hang out with her anyway and say she's "straight" because she makes them cupcakes and does their laundry or something.
Guys don't get it, and so they think this girl is "really cool" and "one of the guys" or "a good drinking buddy." But really, she's lame and uninteresting, and obnoxiously flaunts her unhealthy relationship with "the boys."
Defining characteristics of a twisted sister include - possessive, bland, annoying, posts numerous pictures on Facebook of her and "her boys" at clubs, has evil eyes, talks in stupid "gangsta" voice and takes numerous pics with a "gangsta" face/hand symbol.
Twisted Sister: "Oh muh gawd! This is a pic of me and muh BOYYYYSSSS! You bitches BEST be backin' up off muh BOYS. And I'm bein' RULL cute and makin' my pimp-ass gangsta face! WHUT!"
Normal College Girl: "Bitch, PLEASE. You're just a twisted sister."
Twisted Sister: "Bitch, you CANNOT date Tony! He's on of MUH BOYS!"
Normal College Girl: "Twisted Sister, shut up. He wants to make out with me, not you."