Gah, it took a lot of doing, but I managed to track down the source of this. The name of the game this is from is うちの妹のばあい - Uchi no Imouto no Baai - "Little Sister's Situation".
I added what additional info I could find on the game to the wiki page here. While nothing had been tagged as coming from this particular game previously, there does seem to be a fair number of images by the same artist.
