Likely that small window placement will see very little water. still id look closer at that framing work..

We install Vinyl clad windows in showers frequently. The simple answer is "Execution of waterproofing" and its imperative its vinyl/tempered glass.

There are specific materials and methods to make it seamless and tie in the waterproofing to the vinyl window flange and allowing for expansion/contraction and material to material connections. as well its pertinant the flashing is well executed on the otside and inside the rough in prior to the window install.

Marble jambs and sils are fine as long as you know the long term maintanance and cautions of marble.. It comes down to the waterproofing and the skill of youre tile installers, materials chosen/used, and both sides of the wall working together (Inside tile installer & Outside exterior window installer working together...)- best

"Irregular'' is being kind. Looks like a hack is attempting this. You need to put the brakes on and educate yourself before this goes much further.

mine works GREAT. might i suggest an awning window as then the whole thing opens.

I don't think you have to worry about water damage with the situation you described. Just make sure there is waterproofing up to the window, and then caulk the seam for peace of mind. I recommend having a slight grade on the window sill so water rolls into the shower instead of sitting on the sill - that's what will cause water damage over time.
I have a window in my shower that is lower and larger, in Hawaii (100% humidity and lots of rain), and there was no water damage when we opened the wall to retile.

The interior AND exterior of the window should be water-resistant, well sealed from both sides, lockable, and the glass frosted. I have one like that in my shower and have had no problems. I might reconsider the vinyl window, if I were you. The window should be mounted about shoulder height of the tallest resident. The rest of your materials are expensive. Don't use a cheap window.

I would be more concerned with marble slabs then the window. Marble is a high end look but beware that it is also high maintenance and requires a lot of skill to install properly. Depending on type stains easily, discolors and changes color when wet. Marble requires good waterproofing as it is porus. I would assume you are signing a waiver that installer has no control on the above happening as tilers hate installing marble for the fear of call backs. High end jobs have the invoice padded for such possibilities.

Many bathrooms with windows, it shouldn't be an issue and can also help keep room dry which is a help with the marble.

As for the framing I will assume that is outside wall and the framed within what was there to not possibly harm the exterior finishes. The bottom is just blocking for a possible hot mop? Not neat but it does the job. Remodel framing is never as neat as new construction.

Thank you all for your educated advice.

I accidentally said marble slabs, wrong, it is marble tile. Thank you.