How to remove a granite window sill?
The short answer is maybe, it really depends on what it was affixed with and how thick the granite piece is.  

The key to removing it (or anything) without breaking it will be to have multiple pry points simultaneously, or use a piece of wood (2x4) so that you are prying against the wood which can then spread the strain you are putting on the granite piece.  Now that said, if the whole thing was glued down with epoxy...thats going to be a challenge no matter what.

If you can send a picture of both the top and bottom side of the sill we can get a little more detailed.