What about forgoing light fixture and put in lighted mirrors? Round or oval, as large as would fit might fool the eye.

^^^Or center mirror over sink and add a single pendant on both sides of the window.
Or center a mirror over the sink (or fill the whole area from wall to window and hang a pendant light or one sconce on each side of window - maybe like this double sconce:

I think these mirrors have two sconces, but show what one on the sides of the windows would look like. I probably wouldn't use this style, however.

this almost looks like they had the same problem. I like how they trimmed out the window and mirrors together, filling the whole area between wall and window with mirror and mounting the lights through the mirrors, as you suggested.

It sure does look like the same problem.