Quote Originally Posted by scbubba View Post
Best I can tell (and using Shawshank Redemption as a reference) it is referring to the inmates who aren't necessarily homosexual but they show power and rule over other same-sex inmates with violent sex and sexual activities. They also exhibit brutal behavior outside of sexual acts.

I can't find any references to this outside of Shawshank Redemption, however. So....
That's what it is. Homosexual rapists from the prison are the "sisters".