Sexual relations between individuals ( between people who are Mahram to one another ) who are closely related.
Useful information: If a person commits adultery with one of his mehrams who are united by ties of consanguinity like mother and sister he should be killed under the orders of the Religious Head and the same order applies if an unbeliever commits adultery with a Muslim woman. It has been said in many traditions that awarding one of the religious punishments becomes the cause of' people abstaining from unlawful things; it secures their present world and the Hereafter. and it is more beneficial for them than rain falling for forty days.