I've been debating selling my 26er; full suspension, fork has less than 5 hours on it.
It's a Specialized xc, aluminum frame, I think it's actually a medium but rides like a large (I'm a little under 5'11").
$500
This pic is from the end of July. Your sister is more than welcome to try it out. I'm about 10 minutes from Hubershorne (as the kids call it these days), you're welcome to pick it up and take her riding (wherever), drop it off when your done.