If a person commits adultery with one of his unmarriageable kin (mahram), what is his punishment? And can he repent?

Fornication with a mahram (unmarriageable kin) is one of the gravest major sins and most heinous of abominations. Its prescribed punishment for the fornicator, whether previously married or not, is death, due to the saying of the Prophet, peace and blessings be upon him: "Whoever has intercourse with a mahram, kill him." The fornicator can repent to Allah if he is sincere, regrets his crime, abandons it, resolves never to return to it, follows bad deeds with good ones, and frequently seeks forgiveness and performs righteous deeds. Then, perhaps Allah will accept his repentance, as Allah Almighty says: "Except for those who repent, believe and do righteous deeds. For them Allah will change their evil deeds into good ones. And Allah is ever Forgiving, Merciful."