The part of a woman’s body which must be covered in front of other women (called `awrah) is from the navel to the knees. So a woman should not uncover her thigh even in front of her mother, sister, or close friend.
What the Prophet Said
“A man should not look at the private parts (`awrah) of another man nor should a woman look at the private parts of another woman.” Sahih Muslim #338
Shaykh Mustafa Umar