We know very little about Barney's sister, and this has to be on purpose by the writers. There are two possible reasons why they haven't introduced her character yet: 1) There's going to be an episode where Barney hits on her in a bar before figuring out she's his sister. (This is the most obvious one.) 2) She's The Mother.
Supporting facts: the only thing we've heard about her is that she's "in college." This makes her eligible to have been in the econ class Ted tried to teach, and to be the roommate of the Rachel Bilson grad-student character (both of which are mother-criteria). Also, surely she would be at Barney's wedding, which is where Ted is going to meet her.
That's all I've got, but I'm going with it. This is now my officially sponsored theory going forward (or until such time as they introduce Barney's sister and have her meet Ted without them immediately falling in love).

