Aight, so you want to **** your little sister. You will need to install the official patch.

Install the game normally and run it once. After that, download the patch from https://www.kaguragames.com/product/living-with-sister-monochrome-fantasy/ to your system.

Rename it to something without spaces, like "h_patch.exe".

In steam, go to properties, and set launch options to the following:

~/.steam/steam/steamapps/common/Proton\ -\ Experimental/proton waitforexitandrun ~/Path/To/h_patch.exe; echo %command%

Run the game on steam. It will launch the patch installer. Proceed normally.

Once it finishes patching the game, remove the launch options and launch the game normally.

Congrats, you degenerate pervert.