Islam has declared some relations illegal for marriage and these relations are known as mehrams for women. A muslim man/woman should have knowledge of these relations.
Allah swt says in Holy Quran:
And marry not women whom your fathers married, except what has already passed; indeed it was shameful and most hateful, and an evil way. Forbidden to you (for marriage) are: your mothers, your daughters, your sisters, your father's sisters, your mother's sisters, your brother's daughters, your sister's daughters, your foster mother who gave you suck, your foster milk suckling sisters, your wives' mothers, your step daughters under your guardianship, born of your wives to whom you have gone in - but there is no sin on you if you have not gone in them (to marry their daughters), - the wives of your sons who (spring) from your own loins, and two sisters in wedlock at the same time, except for what has already passed; verily, Allâh is Oft¬Forgiving, Most Merciful. (An-Nisa 4:22-23)
And similarly Rasool salal laho aliehi wasalam has forbidden aunt and her niece in wedlock at the same time. All muslim men and women should well aware of these relations declared illegal for marriage by Allah and His Rasool salal laho aliehi wasalam
