Susan,
In my view, you don't need a background. I think the compositional weight is now balanced between the background and the light shirt area. Fill up that background and you're going to have to do something with the shirts, or they'll look like an "unfinished" part.
I don't know how difficult it is to make alterations in the particular medium you're using, but I'll mention two or three areas that I think would profit from attention.
If, as Josef recommended, you look back and forth repeatedly between photo and drawing, you should discover that in the drawing, the head of the girl on our left isn't as "tall". It's difficult in a small reference to see where the space was lost. I think the chin is small. It could be enlarged and "squared up" slightly. On the top of the head, I'd lighten up the headband across the top and then as it moves to the right, let it darken in value and get "lost" in her hair. I think that would add a bit of height on that end.
I had thought the curve of that same girl's jawline was too rounded, and I think that enlarging the chin size and then "aiming" the jawline at the new, lower base will take care of that.
I'd have a look at shadow and halftone areas in that same girl's neck. Don't let anything get too light. The whole area is in shadow. Also, try to carve out and define that interesting dark on the right side of the neck, which drops down from the jaw, down the center of the windpipe to the pit of the neck, and then to the right.) Don't make it too dark, though.
Lastly, where the girls' heads meet, darken the halftone on the left side of the forehead on the girl to the right. This is needed to round out the form. A similar darkening on the left side of the other girl would help in the same way.
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