Plain old white vinegar. Vinegar kills mold spores. You might consider a dehumidifier. Finding the source of moisture will certainly help.
Clean the sill, then spray Concrobium mold control (available at home and hardware stores) to the moldy areas, carefully following directions.
If windows are wood, you may need to sand, re-stain or paint, then seal with 2-3 coats of polyurethane.
Run a dehumidifier in your home, set no higher than 40% humidity. Close all doors and windows to this area when dehumidifier is operating.
Run bath fans during and for 20-30 min. after showers. Use bath linens that have never been exposed to fabric softeners.
Squeegee shower walls and ceilings after showering to remove excess water.
Run kitchen exhaust fans and ceiling fans when cooking/baking/microwaving.
Check gutters and downspouts for leaks. Repair as needed.
Check the window "lintels" below the exterior of the windows, to insure no mortar is missing between bricks, which can collect water. Replace mortar if needed.
If windows are newer, your windows may not have been installed with proper pitch to the sills, to insure adequate drainage. If still under warranty, call the installers to correct the problem.
If your windows have "weep holes" , make sure they are clear of spider webs and dust, using a paper clip or similar.
Thank you very helpful