Mahram means men a sister cannot marry. Her father, grand dads, sons, brothers, first uncles, sisters' husbands and husband.
And vise versa for men.
Asalamo`Alaykum Wa Rahmatullahi Wa Barakaatuh,
JazakAllah khair , i just wanted to complement the answer of the sister with the verses inshAllah to encompass the whole list of mahram
Quran 4: 22-23
And do not marry those [women] whom your fathers married, except what has already occurred. Indeed, it was an immorality and hateful [to Allah] and was evil as a way.
Prohibited 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 [milk] mothers who nursed you, your sisters through nursing, your wives' mothers, and your step-daughters under your guardianship [born] of your wives unto whom you have gone in. But if you have not gone in unto them, there is no sin upon you. And [also prohibited are] the wives of your sons who are from your [own] loins, and that you take [in marriage] two sisters simultaneously, except for what has already occurred. Indeed, Allah is ever Forgiving and Merciful.