It's a little bland to me as is, but if you were going for a more serious pose (because of the serious faces) this is how I would edit it:
I first cleaned up the blemishes on her face, as best as I could from just saving this file. Then I warmed up the colors (I use PSElements, and use "adjust color for skin tone" and then adjust it from there manually) Then I added the detail brush to both their eyes to pop them out more, and finally increased the contrast on the whole picture.
I think the overall effect is that the lighting is warmer and more golden, and their serious faces match the "mood." Otherwise with the cool tones, I don't think it's a flattering light for either of the girls, with that expression. I like the posing, and I like that you including their hands. I might have the little girl lean into her sister (?) more because it looks like she's trying to get away, but I really like how you have her hand on top of her sister's.
I know it's easy to go over the top with photoshop, but I think it's vital to always at least clean up a woman's blemishes. That, to me, can make or break a photo.