My first impression was that I loved this photo. Now I'm not so sure, and would be interested in some other opinions. Thanks!

The thing that really bugs me about this shot is the way it's framed / cropped. I don't mind having both of their faces partially obscured in general, but any eye that's in a portrait... needs to be all the way in. The problem here is that I can see parts of one eye on each of them, and it's very distracting. One eye per kid or both, but not 1.5 eyes.
;)

Also, the contrast is a little flat.

I would have to agree with PuckJunkey on this one... the 1.5 eyes are distracting, imo. Perhaps since you are cropping things, you might want to crop in a little closer on the tops of their heads as well... It would draw more attention directly to their faces and eyes... particularly to their expressions. You've done a great job of capturing the expressions and I love the duality... one girl looks serious, the other, playful. It's gives a lot of meaning to identify them as sisters too. If you do decide to crop it, post the revision back up, I'd like to see it. This is a pretty good photo. *smile*

Thanks to both of you... how about this one? P.S. I actually meant to only get one of each eye in the original photo, but I can never seem to get it right, since my eyepiece leaves out like 5% or something. Practice, practice, practice! ;)

Also Puck, do you think I should add density to correct for the flatness? Thanks!

I like the cropped version, it works... at least in my opinion it does. You should perhaps do something about the flat color... play with the levels and curves and color balance... if you don't like something you can always cancel or undo.