Any woman with whom a man has a relationship (of blood or fosterage) that precludes marriage, is considered a Mahram to him.
“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 go 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.” (An-Nisa 4:22-23)
Mahram women include his mother, grandmother, daughter, granddaughter, sister, aunt, grandaunt, niece, grandniece, his father’s wife, his wife’s daughter, his mother-in-law, his foster mother (the one who nursed him), foster sisters, and any foster relatives that are similar to the above mentioned blood relatives as the Prophet (SAW) said, “What is forbidden by reason of kindship is forbidden by reason of suckling.” (Al-Bukhari)
All the man’s female relatives mentioned in these two verses are considered his Maharim, because it is unlawful (haram) for him to marry them, except the wife’s sister mentioned last, who is not a Mahram because he can marry her if he divorces her sister, or if she dies. Reciprocally, if a woman is a Mahram to a man, such as her brother, her father, her uncle, etc. then he is a Mahram to her. All other relatives are considered non-Maharim and they fall under the category of strangers to her, except one’s wife or husband who is also called Mahram.