What Is a Woman’s ‘Awrah?
Question

What is the definition of the ‘awrah between a woman and her brother? What is the ‘awrah between a girl and her mother or sister?
Praise be to Allah.Woman’s ‘awrah in front of mahrams
The ‘awrah of a woman in front of her mahrams such as her father, brother and nephew is her entire body except that which usually appears such as the face, hair, neck, forearms and feet. Allah says (interpretation of the meaning): 
“…and not to show off their adornment except only that which is apparent (like both eyes for necessity to see the way, or outer palms of hands or one eye or dress like veil, gloves, headcover, apron), and to draw their veils all over Juyubihinna (i.e. their bodies, faces, necks and bosoms) and not to reveal their adornment except to their husbands, or their fathers, or their husband’s fathers, or their sons, or their husband’s sons, or their brothers or their brother’s sons, or their sister’s sons, or their (Muslim) women.” [al-Nur 24:31] 
So, Allah has permitted a woman to show her adornment in front of her husband and mahrams. 
What is meant by adornment is the places where adornments are worn: the place for a ring is the hand, for a bracelet is the forearm, for an earring is the ear, for a necklace is the neck and chest, and for an anklet is the leg. 
Abu Bakr al-Jassas (may Allah have mercy on him) said in his Tafsir: 
“The apparent meaning indicates that it is permissible to show one’s adornment to one’s husband and to those who are also mentioned in the verse, such as fathers etc. It is well known that what is meant is the places where adornments are worn, which are the face, hand and arm… this implies that it is permissible for those who are mentioned in the verse to look at these places, which are the places where hidden adornments are worn, as it says in the beginning of the verse that only outward adornments may be seen by strangers (non-mahrams), but the husband and mahrams are permitted to see hidden adornments. It was narrated from Ibn Mas’ud and al-Zubayr that this refers to earrings, necklaces, bracelets and anklets. 
This applies both to the husband and to the others who are mentioned along with him. The general meaning implies that it is permissible for those who are mentioned to look at the places where these adornments are worn just as it is permissible for the husband.”
Al-Baghawi (may Allah have mercy on him) said: 
“The words of Allah, may He be exalted, “and not to show off their adornment”, mean that they should not show their adornments to a non-mahram. What is meant here is the hidden adornments, as there are two kinds of adornment, hidden and apparent. Hidden adornments include anklets, henna on the foot, bracelets on the wrist, earrings and necklaces. It is not permissible for a woman to show these, or for a stranger (non-mahram) to look at them. And what is meant by adornment is the place where the adornment is worn.”
It says in Kashshaf al-Qina’ (5/11): 
“A man may also look at the face, neck, hand, foot, head and calf of a woman who is his mahram. According to this report al-Qadi said: It is permissible (to look at) that which ordinarily appears such as the head and hands up to the elbows.”
These mahrams differ in the degree of closeness and risk of fitnah (temptation). Hence a woman may show to her father what she may not show to her husband’s son.
Al-Qurtubi (may Allah have mercy on him) said: 
“Allah mentioned husbands first, then He mentioned mahrams, and described them all as equal with regard to showing the adornment. But they may differ according to what is in their hearts. There is no doubt that for a woman to uncover in front of her father or brother is more safe than to uncover in front of her husband’s son. The extent of what she may show may differ, so she may show to her father that which it is not permissible for her to show to her husband’s son.”
Woman’s ‘awrah in front of other women
What is established among the jurists is that the ‘awrah of a woman with another woman is the area between the navel and the knee , whether the woman is her mother or sister or is not her mahram. 
It is not permissible for a woman to look at the area between the navel and the knee of another woman, except in cases of necessity such as medical treatment and the like. 
This does not mean that a woman may sit among other women with all of her body uncovered except the area between the navel and the knee. No one does that except women who are promiscuous and negligent, or immoral and evildoers.
 The words of the scholars, “The ‘awrah is the area between the navel and the knee” should not be misunderstood, because this does not mean that this is how women should always dress and show themselves among their sisters and friends. No wise person would accept that and it is not what the fitrah (sound human nature) calls for. 
Rather the way a woman should dress among other women is in clothes that cover properly and express her modesty and dignity. She should not show anything except that which appears when she is working and serving others, such as the head, neck, forearms and feet, as we have mentioned when discussing mahrams above. 
The Standing Committee for Issuing Fatwas has explained what a woman is not permitted to uncover in front of her mahrams and other women. We have quoted this in the answer to question no. 34745 . 
We ask Allah to guide us and you. 
For more, please see these answers: 43289 , 26304 , 102187 , 310812
And Allah knows best.