Original "chibi" styled animations of Imouto sae Ireba Ii. posted on the anime's official Twitter channel. Also included on the BD/DVD releases.

Every day is full of fun. But something is missing. "My life would be amazing if only I had a little sister. Why don’t I have a little sister...?" These are the musings of little sister-lover and novelist Itsuki Hashima, who only writes works featuring little sisters. Around him gather a number of unique people: genius author/pervert Nayuta, female college student Miyako, illustrator Puriketsu, and the brutish tax accountant Ashley. Each of them hold their own worries, but live their peaceful daily lives while writing novels, playing video games, drinking alcohol, and filing their tax returns.

[Source: Crunchyroll]

Two additional episodes of Imouto sae Ireba Ii. (ONA) released exclusively on the BD/DVD volumes.