Torikago no Naka no Kimi

An exemplary student with excellent grades, Souma was abandoned by his mother when he was a child and his aunt and uncle took him in. They are a wealthy family, but the uncle is completely uninterested in Souma, and his aunt loathes the fact that they have to take care of him. The only one who needs Souma in this unwelcoming household is his cousin Sakuya, who becomes very important to Souma. But 12 years later, when Sakuya learns about Souma's plan to become independent, he takes Souma's body by force...

(Source: Taiyo Tosho, translated)