I can't tell if Don is like that because the artist likes her the most out of the three, or if they thought it'd be funny if a goofball like her was the most detailed out of them.
When Sakuya refers to "Imouto-sama" she probably talks about Flandre. (Just for context)
Also it actually reads "Masturbating to pics of big sis Sakuya" or "Masturbating to pics of Sakuya onee-sama.
The first one is absolutely correct, the second one, it's up for debate. If you check the comments in Pixiv, people are wondering whether there's an implicit comma or not (as in 「さくや姉様の写真でオナ」—Masturbating to pictures of Sakuya-nee-sama— or 「さくや、姉様の写真でオナ」 —Sakuya masturbates to pictures of nee-sama). Given the subject at hand and how the meme usually goes, I would imagine the latter would be the most likely option.
