Take the wood out, install bullnose to match the tile, or make a wrap-around trim from a marble sill. Talk to a tile guy.

^^^^, it will rot if left unchecked.

Wow that was supremely poor planning. At least use azec if you were attempting this look. At this point the jamb extension comes out and has to get replaced with something rot free- marble, corian, azek

That wood window needs replacing with fiberglass or vinyl. You aren't going to bandaid your way out of the poor design, and incorrect waterproofing situation without doing a full redo of it all with a different contractor.