Do you have a picture or can you give us the size of the tile? Sometimes it is possible to remove the sill altogether and use a small piece of trim, like a quarter round, for transitioning into the tile. This gives a little more room for the tile to extend. Alternately, You can use the countertop material to fill the between the sill & countertop. This works better IMO if the tile and countertop are similar in color.