Bump!

Is the only window sill that needs to be "finished" that upper wide short elongated one?

That one It looks as if it is "incomplete" because something was wired into the space and then removed. Do you know what was there? and/or Do you want to use the wires for any reason ... such as to hang a light fixture or something else electrical there? If not, then the fix is to hide the wires before finishing the window sill with dry wall.

The objective is to be able to render the wires safe even if there is still power on them and be able to fish out the electrical wires without cutting into any framing should you ever need to do so -- this way only the drywall would need to be cut to do that:

Check the hole the wires come out of to see how large it is -- don't touch the ends of the wires yet. First Turn off all electrical in your home to make sure this circuit is off at least long enough to make the wires safe to hide them in the wall. To do that, you'll need to wrap the end of each wire individually with electrical tape.

Tie the wrapped wires together using one end of a foot long length of fishing line. Tape over the fishing line to keep it in place attached to the wires.

Secure the loose end of the fishing line so you can keep it out of the hole as you push the wires back into the hole they're coming out of. You might tie something such as a kitchen tool with a hole in the handle to it -- something that will not fit into the hole.

Tape over the hole with the one end of the fishing line still visible. Then you can cut your weight/tool off the visible end of the fishing line. You'll need to make sure the loose end of the fishing line stays "up top" -- maybe staple and/or tape it to the unfinished window sill.

Now you can finish your windowsill with drywall and paint.

If after watching some You Tube vids on how to do dry wall, you decide, for whatever reason, you need/want an easier way, then just hide the "damage" instead of repairing it.

Once the wires are hidden with loose end of fishing line secure, just paint the unfinished surface. Then you can take some measurements to determine the size of the space you need/want to hide and start looking for a basket or something else that size that can be left on that windowsill.

thanks suezbell, it was a wall AC.

I had the wires disconnected and capped off in the attic. They will be used for a different project. The wires in the picture are not connected to anything on either end.

The part I was struggling with, is it seemed if I do anything to the center window, I have to do the same to the side windows.

I was not sure if I should frame the inside with drywall pieces or wood. Currently they used 1/8” thick pieces and painted those.

Thanks for any suggestions!

As long as the two side windows match, you have symmetry and could do something entirely different with the center window; however, unless you want to make that the focal point of the room, "same" would be much better.

Try to find sheet rock (or plywood) the same thickness as those pieces removed -- or use some slightly slightly thinner -- and attach those to both upper and lower areas.

The bottom should be easy enough smooth with sheet rock mud over and around the replacement piece you add, especially if you've used a wood putty or caulk to fill in where the pieces meet and smoothed that and let it dry first. It is only if the thickness of your filler board it considerably less that you'd need to add extra thicknesses sheet rock mud to level the surfaces.

Same applies to the top of the niche ... except that IF you need to add extra thicknesses of sheet rock mud to level that top area, you may need to apply it in thinner layers and allow each layer to dry and adhere to the surface before applying the next layer -- otherwise gravity could undo your work.

All three of the windows next to the ceiling are so high that, unless you're unusually tall, there is no "view" thru them anyway ... just soffit.

What you will see mostly is the upper part of the niche. If your repair of the upper part doesn't turn out as well as you'd like (or if you're worried it will not), then I'd think you could choose to add a thin piece of smooth sanded lauan (really thin plywood) to the top of each (3) upper window niche and finish around the edges of that.

... The difference in the (both smoothed) surfaces, once painted, would likely be unnoticed.

... Since the bottom of the window frame cannot be seen anyway (because of the height of the window and the depth of the window sill), the fact that there is a lauan depth less of top window frame visible above than is visible below should not even be noticed.