Note: This is NOT the Ano Hi Mita Hana no Namae wo Bokutachi wa Mada Shiranai dj. They have the same name but are completely different.

Basically Guy likes his cousin, sees her with another guy. Unable to help himself he ends up forcing her to have sex with him. Feels ashamed and runs away afterward. Doesn't go back to see her for a long time. They grow up. One day he gets an invitation to her marriage. Returns. Girl crawls into his bed at night and forces him to have sex with her because she's always loved him. Guy resolves to ask for the termination of her current engagement and her hand in marriage. Only to find out that it was all a trick by the family, who had conspired with the girl to get the two together.

(There are cultures where cousins are allowed to marry and not considered as incest. Thus I don't think the incest tag belongs here.)