It really annoys me that she was gone for SIX YEARS and neither Peter nor Derek care to ask where she was all that time. She claimed she couldn't find Derek and thought he was dead or some bullshit, but he was in Beacon Hills the entire time. WHO WOULDN'T CHECK BACK AT THE HOUSE WHERE THE FIRE HAPPENED TO SEE IF YOUR BROTHER WAS THERE?? THAT WOULD BE THE LOGICAL THING TO DO. AGH.
Also, she was kind of a bitch in this episode. So not feeling her anymore.

I think she is just trying to get everything together. We don't know how long the Alphas had her, plus, she thought her brother was dead with the rest of her family, there is no reason she would have looked for them. Besides, Causing Derek more pain is the last thing any of us want. If she does turn out to be evil I think it would be a dumb move and too clique. How many times have they done the Evil-sibling shit in TV shows/ Movies?