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.
Please apply your seal~What's that?A letter for you, from Imouto-samagriiin
The 26th of December is
Boxing Day!
A day to give gifts to the employees and couriers that delivered Christmas presents!
Now that's a pretty good "scoop present"Recently, in Gensoukyou—
I would love to know the details!
Of course, miss Tengu's stories—
I borrowed miss' photo book of various things about Gensoukyou—
—it was an amazing book!
Having the job of revealing the truth is—
—but I wouldn't be able to—
The color of the ornament is—
There's this food called naan—
—I would like to try it—
The first letter of each line reads as "Sakuya masturbates to pictures of nee-sama"
