I think my torso length is fairly avg. and I'm 5'5" tall. However, with my 7.2 Trek FX WSD bike, I find that the seat sits too close to the handle bar. I've pushed the seat back as far as it will go, but I still feel a little cramped. You might have your sister compare the WSD and the unisex bike. Other than that, Trek makes a great bike and I liked the FX much better than the hybrid.

Good Luck