Saikin, Imouto no Yousu ga Chotto Okashiinda ga. OVA
This OVA is divided into two parts. First one is about Torii Shoutarou following his younger sister on a date. Second one is about Christmas Eve, which Mitsuki, Yuuya and their friends spend at Kanzaki's house.
Bundled with the limited-edition volume 7 of the manga.