All three answers are wrong. cover all the windows top to bottom in sheer gathered tight on rods top and bottom. The sheer lets the light in and hides the view to the "other room", which like it or not is a little weird. That is ALL I would do to those windows and dress the ones that actually are real windows to take attention away from the ones that aren't.

Like Gantt suggested, but not a "gathered sheer". I like the sheer valance in your pic very much, but would make it much bigger, like a bottom-up shade.

Here is a full panel of the sheer curtain. I still need to iron out the wrinkles but do you prefer this over the shorter valance?

Another view

I like this one from groveraxle.

I like that, Groveraxle! And soooo true about men!

@TeeCee, I once had a similar situation and used the method @GanttsDecorating suggested. Fabric needs to be twice as wide as window so when it's gathered it's a softer look that blocks view of other room, let allows light if you need it!

TeeCee, buy your paint ahead of time. Match the color that's on the window sash. SEND HIM FISHING. He'll be gone all day. When he returns with his catch, freshly painted woodwork will be his reward.

I would want to either open it up, or put in a wall.