Being twins, it seems that one of them was given for adoption directly after birth, as the mother was a teen who could not raise twins on her own and the father was not around. They discover by accident (I haven't figured out how yet, but there should be many possibilities) what had happened and thus that they have been having a forbidden incestuous relationship for so many years.
At first they are shocked, they even decide to separtae, although they love each other very much. But they can't stop loving each other and can't stop seeing each other, as they are brother and sister after all. Each meeting is a torture for them, the inner conflict is exhausting. But slowly they overcome it until they finally make love, this time aware of their kinship, which makes it all the more exciting for them.
At first they are shocked, they even decide to separtae, although they love each other very much. But they can't stop loving each other and can't stop seeing each other, as they are brother and sister after all. Each meeting is a torture for them, the inner conflict is exhausting. But slowly they overcome it until they finally make love, this time aware of their kinship, which makes it all the more exciting for them.