Dear Dianna,
I have just a couple of comments to offer. It looks as if you took the resource photograph with a flash attachment. If you haven't already had a chance to read the many helpful comments on lighting subjects, you can go to the "Lighting and Photographing your Subject" section for some great reading
http://forum.portraitartist.com/foru...s=&forumid=47. Without good source material, your outcome will always be severely compromised. Even with the best of resource photos, the shadows will appear falsely dark, especially in the nasal labial folds connecting the corners of the mouth with the nose. Only you can compensate for this in your portrait.
Despite the limits of your source material, you have a good command of very subtle modeling of value, which you should be able to readily translate into your next drawing or painting. Your darks look sufficiently dark to me. I also don't feel that with the crop and composition of this portrait, that you need a background. There's a lot going on here, and your negative spaces are working well.
With regard to design/compositional issues, watch for tangents in your painting, as they can easily overpower your center of attention and control the viewer's eye in unintended ways.(See diagram below.) There are three major tangents here that I see: first, the place where the foreward child's hair exactly touches the curve of the older child's jaw; second, the sharpness of the plaid in the shirt, which touches the foreward child's upper lip, as well as entering her mouth at the top of the lower lip. Third, the shape of the neckline is also more repetitive than you would want, a continuation of the shape of the smaller girl's profiled nose. Fortunately, tangents are usually pretty easy to fix.
Last (although it should probably be first) there is the central point to the painting, which is the center of interest. There are lots of things that shape this for the viewer, but among the most powerful are tools like high contrast, and sharp edges (other tools that would apply to color include saturation, and discordant color). On my monitior, the center of interest is older girl's teeth. Because they are rendered with individual shapes, high contrast, sharp edges, and highlights on the gums, they are the most prominent part of the portrait to me. I find that it helps me, when working from photos, to keep reminding myself of the focus of my piece, because photos are misleading in the detail they want to show to you.
Best wishes,