There are no sloped marble sills. You create the slope with the flat pc of marble when you set it.
When necessary I make very narrow pcs from the floor tile and create a sloped transition down to hardwood or carpet.

There are prefab niches but most installers fabricate them to the size they want. Construction details are dependent on the tile underlayment materials your planing on using.

As long as they are built and waterproofed correctly there should be no problems.