In Islam, the term Mahram means anyone with whom it would be impermissible to marry because of their close ties of blood, breastfeeding or marriage relations. In addition, when a woman/man marries his/her potential spouse, they become a mahram to one another by virtue of the marriage contract.
Allah says (interpretation of the meaning) regarding women who are mahram to men:
“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 mothers who gave you suck, your foster milk suckling sisters, your wives’ mothers, your stepdaughters 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”.[An-Nisa’ 4:23].
Likewise, Allah also says (interpretation of the meaning) regarding men who are mahram to women:
“And tell the believing women to reduce [some] of their vision and guard their private parts and not expose their adornment except that which [necessarily] appears thereof and to wrap [a portion of] their head covers over their chests and not expose their adornment except to their husbands, their fathers, their husbands’ fathers, their sons, their husbands’ sons, their brothers, their brothers’ sons, their sisters’ sons, their women, that which their right hands possess, or those male attendants having no physical desire, or children who are not yet aware of the private aspects of women…..” (24:31)
So to break it down further, a mahram becomes one by:
1. Blood relations: Her father, grandfather, great-grandfather, etc., and her son, grandson, great-grandson, etc., her paternal and maternal uncles, her brother, brother’s son and sister’s son.
2. Breastfeeding: the brother and husband of the woman who breastfed her.
3. Marriage relations: The mother’s husband, the husband’s father, grandfather, etc., and the husband’s son, grandson, etc…..
Therefore, for a woman, hijab/niqab is not obligatory in her mahrams’ presence. Excluding her husband, the awrah of a woman in presence of her mahram is her entire body expect her face, hair, neck, forearms, legs below the knees and feet. As for her husband, there are no restrictions.
We need to also keep in mind that although these close relatives are mahram to her, she still needs to maintain limitations in her modesty and keep herself away from displaying herself in a way which may cause temptation (as we see prevalent in our present time). Therefore, her clothes should not be tight, see-through, or uncovering her awrah.
In addition, a woman’s mahram must serve as her legal escort when she is travelling within the time and distance required in Islamic Sharia.
As for men who are non-mahram to a woman, such as her cousins, uncle in law, brother in law…..she is to make sure she wears proper hijab in their presence. Both non mahram men and woman should avoid seclusion with each other, unnecessary talks/chats and glances as well as shaking hands. And although in some customs, one can not not marry his/her cousin, in Islam, it’s permissible, and therefore, he can NOT be her mahram under any circumstances and one should not make permissible what Allah has forbidden or forbid what Allah had made permissible.
Allah’s religion is Perfect. Islam teaches us purity and how to protect ourselves from falling into sin and undesirable situations. So to have such restrictions, we are safeguarding ourselves, our marriages and society as a whole.