User blog comment:The Shadow User/Solana Vakarian/@comment-74.233.28.15-20130212225746

A. I'm pretty sure that is his sister, due to it being another call back to dragon age (like Oriana being Cousland's sister-in-law and Miranda's genetic twin). So it's a a fairly safe assumption that she is his sister.

That said, I think Garrus didn't mention his rough family relationships because it bothers him on a deep, personal level. Thus, airing it out to Shepard could make it prominent enough to distract him from the mission, which is is a bad thing. No distractions from the suicide mission.

On the flip side, it would've made some nice background points in ME3 or for a romanced Shep.