A Muslim woman should cover her hair within the presence of stranger men. She can only be uncovered within the presence of her father, her brothers, her children, her uncles, her father-in-law and other women.
Allah the Almighty commands:
“Tell the believing women to reduce some of their looks, and cover their private parts. Tell them not to uncover their beautiful parts except what is naturally seen. Tell them to put their scarves onto their chests and not expose their beautiful parts except to their husbands, their fathers, their husbands’ fathers, their sons, their husbands’ sons, their brothers, their brothers’ sons, their sisters’ sons, their women, those in their possession, those male attendants that have no sexual desire to women or children who are not yet aware of the private aspects of women. (…)” (An-Noor / The Light 24:31)