Close the window up. Or do a transom window, much higher on the wall. None of this is cheap or easy. And it adds 5-10K to the average 50K 5 pc master bath remodel.

My Dad has a window inside his shower, but it's very high up on the wall, at about 72" off the floor. He can reach it to open (it's a slider). The window was old so he had it replaced with a new vinyl one, and the tile pro waterproofed and tiled it along with the rest of the shower. He didn't change the shape or size of the window, so it was pretty simple. If you are changing the shape, size and location of the window, it's more complicated because you will have to redo the wall and it also involves the outside of your house: brick, siding, etc. A window replacement company could come out and give you a proposal. When I did something similar, the window replacement company handled the brick work outside. Make sure your tile pro knows how to waterproof a window.

+1 to Sophie's advice. Shorten the window considerably and move it up and out of the wet area as much as possible.

birdkkk, I'm doing the same thing you are and had the same question. We had 4 contractors out and only 1 said to shorten the window. The others recommended removing the glass and putting obscure glass in it as well as tempered. They said if it is sealed properly, there will be no problem leaving it. I decided to get a brand new vinyl window with the rain glass since the window is 26 years old and has an aluminum frame. (more cost effective). Also, the wooden window sill will be replaced with the tile framing the inside of the window.