My Sister, My Writer

Yuu Nagami, a high school student aiming to become a light novel author has a little sister named Suzuka. Suzuka is an excellent student with good grades and great looks, but she's very cold toward her older brother. Every year Yuu keeps failing at the preliminary rounds of the light novel contest, but one day Suzuka reveals a shocking truth. The flirty sibling romantic comedy that Suzuka wrote has won the light novel contest! To make matters worse, Suzuka doesn't understand anything about light novels or moe so she begs Yuu to stand in for her under her pen-name, Towano Chikai!

(Source: Crunchyroll)