i don't care how few miles she is going...if it is the pacific northwest i remember disc brakes will be a plus at stopping time! (btw...my commute there was hilly)
i am a flat bar girl as well...i road ride & mtbike so i know the difference and i prefered a flat bar (surly 1x1 torsion bar) on my monkey. wanting to try a midge just for kicks tho!
does she have a hilly commute? which one is lightest? does she show an interest in any other kind of riding? (MTBing**********?) which one would help that transition? test ride, test ride, test ride, test ride!!!!!!
good luck...and happy commuting to your sis (you are being a great bros helping her out!!!!!)
***oh...did i mention test ride**********