I treated it in exactly the same way as the door. In my case, the window was screwed to the frame with the front flush with the cladding. I then attached a piece of DPC down each side, which overlaps the window frame, and another piece over the window which overlaps the DPC down the sides. This was tacked in place with a staple gun and then sealed down the side next to the window with some sealant. No need to bother with sealant above the window. The timber that frames the window externally is narrower than the DPC (150mm from memory over a 200mmDPC) and then the shiplap just butts up to it.
In your case, as there is no OSB sheathing, the window frame would be flush with the internal frame and you might want to take the shiplap right up to the edge of the DPC where it overlaps the window, and then attach the external frame over the top of the shiplap. You could always run a narrow batten/bead around end of the shiplap to cover the end grain. HTH