You didn't mention if the stain and varnish was applied to an existing prefinished cabinet or a new raw (unstained) cabinet. The following will work for a raw cabinet recently stained and finished.
There is still some varnish on the surfaces. You will need to carefully sand some more to remove the remaining brush marks and bristles. If the marks and bristles are a part of the stain, left behind during the staining, then do some more sanding and restain. This should take care of it.
If the cabinets were prefinished then they will need all of the new stain stripped off and be restained. Try some solvents to help remove the new stain; lacquer thinner, xylene or acetone. The idea is to remove only the new stain and not damage the old finish. Experiment a little on the back of a door or other hidden area. Using a stripper will be risky as it could remove the old finish as well as the new, creating more work.