Koi Kaze - Season 1 Episode 12 Spring Thunder

Nanoka breaks their promise and comes to Koshiro's apartment to see him. She offers to make him a meal, but his refrigerator is empty so he goes out to pick up some things. In the meantime, Kaname drops in and is shocked to find Nanoka there. She tries to make Nanoka go home by telling her that she is going out with Koshiro. But Nanoka refuses, saying that her feelings for her brother are greater and she insists that Kaname leave instead. Kaname encounters Koshiro on his way home and tries to talk sense into him, but he won't listen. Kaname says that she will not interfere any more. The siblings decide that they really do love each other and want to be together no matter what. They spend the night together as lovers.