User blog comment:Western Gen/Speculations for Season 12/@comment-5431899-20140808115903

Credits to Reddit:    Canon answer: Lopez killed her.

Real answer: Her character development wasn't going anywhere and she didn't really bring anything to the show other than "Grif's sister is a slut and so now there's a slut at Blood Gulch". I'm pretty sure they decided to write her out instead of fleshing her out further.

Sorry, but I doubt Sister will be coming back for anything other than maybe a one-off joke or two.