Ane Oyako de Iku!

Kudaka Yuushi loved his sister Yoriko since childhood. And so it was quite a shock to him when on one of a family meetings he found out that she was getting married and was already pregnant. Because of that, their parents disowned her and he lost contact with her. Disappointed with his feelings for her, he long tried to cut them off and bury them. Spoiler Over decade later Yuushi became a popular novelist and lives alone in the city. And just when he thought he had finally reached peace of mind, his doorbell rings. To his surprise, at the door he finds his sister standing together with her daughter Midori. They left their home because Yoriko grew fed up with her husband's repeated affairs and now they need a place to stay. Because of these circumstances they end up living together, until one day, Yuushi witnesses his sister calling her husband and trying make up with him. However, the husband has no intentions of stopping the affairs at all, and instead of reconciling they end up fighting even more. In the end furious Yoriko hangs up and declares: "If it goes like this, I'll cheat on you too!!!" Yuushi is absolutely stunned upon hearing this sudden and unexpected declaration from Yoriko. And when he asks her who she plans to have an affair with, Yoriko for some reason chooses him. Could this be a chance to win his sister back from her unfaithful husband? Thus, Yuushi began taking action to reclaim his sister from her husband, both physically and emotionally.