I think the skin looks a little flat in the first couple of images of the "sister" character. While the shadow areas are there, the highlights are either not there or they're plain white. In the last image you posted (#27), the lighting/shading of the skin looks a lot better imo, but the face of the girl seems like 10 years older (not around 18), i don't know if its those lines at the sides of the nose or the general expression of her face.
Overall, i'd say a little bit more depth (skin texture wise) goes a long way to make the game more engaging, but they do look nice.
Aside from that, im a graphic designer and im looking to get involved in the game developing world in some form, so if you need a hand with anything (specially graphics related), let me know.