Consider some recessed lights as well if you are worried it won't be 'bright' enough.

If your sconces are of the standard 60watt (850 lumen) variety, I would add a recessed of at least 850 lumens above each sink as well. Otherwise, you will need at least 1200 lumens on each side of each mirror without the the recessed. The color of the walls, ceiling, etc also need to be considered. The opacity of the diffuser or shade, and your age also effect the amount of light needed.