Which women are forbidden for marriage in Islam?

Who are the unmarriageable female relatives in Islam? Can a person marry his mother’s sister?

 There is a group of females called “muharramat” in Islam that are forbidden for marriage because of kinship. They are stated in the 23th ayah of Surah al Nisa:

“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, Allah is Oft Forgiving, Most Merciful. “ (Surah al Nisa; 23)

“And all married women (are forbidden unto you) save those (captives) whom your right hands possess. It is a decree of Allah for you. Lawful unto you are all beyond those mentioned, so that ye seek them with your wealth in honest wedlock, not debauchery. And those of whom ye seek content (by marrying them), give unto them their portions as a duty. And there is no sin for you in what ye do by mutual agreement after the duty (hath been done). Lo! Allah is ever Knower, Wise.” (Surah al Nisa; 24)

As clearly mentioned in the ayah, your mother’s sister that is your maternal aunt is “mahram” to you. In other words she is an unmarriageable relative to you who is eternally forbidden relative for marriage.

However, as regards your “step aunt”, she is not “mahram” to you. Because she is not your “birth aunt” mentioned in the forbidden group in the ayah.