Cracked Window Sill [upd] -

If the crack is wide, use a two-part wood epoxy . This material bonds to the wood fibers and creates a waterproof, structural seal that won't shrink.

Remove any loose paint or debris from the crack using a wire brush or the edge of your putty knife. If the crack is deep, use a vacuum to remove dust. The cleaner the surface, the better the filler will adhere. cracked window sill