Praise be to Allaah.                      Firstly:  
          There is nothing wrong with a Muslim woman mixing with her      mahrams, including kaafirs who are forbidden to her in marriage (because of      close ties of blood or marriage, such as the father-in-law), if there is no      danger of fitnah from them. The same applies to kaafir women among her      relatives or others whom she has to mix with for some reason, such as her      husband’s mother. The Muslim woman should call them to Islam in ways that      are better, by showing the best attitudes promoted in Islam, such as      speaking and acting kindly, whilst adhering to Islamic rulings such as not      being the first to greet them and not feeling love towards them in the      heart. 
          With regard to the way she should dress in front of them, it      is permissible for a Muslim woman to uncover in front of her mahrams and      womenfolk as much as is customary among people of religious commitment and      good manners, such as uncovering the head, neck, forearms and part of the      calf. 
          Whether the women and mahrams are Muslims or kaafirs, there      is no difference in the ruling. 
          But it is not permissible for a Muslim woman to wear short      clothes in front of them, or thin (see-though) clothes, or tight clothes      that show the shape of the ‘awrah. The scholars of the Standing Committee      said: 
          A woman should be modest, even if no one can see her except      other women, she should not uncover anything more than is customary or needs      to be uncovered, such as going out in the kind of clothes that she wears      when doing housework, showing the face, hands and feet and the like. That is      more concealing and farther removed from any doubt or suspicion. 
          Fataawa al-Lajnah al-Daa’imah,      17/288, 289 
          Shaykh Muhamamd ibn Saalih al-‘Uthaymeen said: 
     With regard to her      mahrams what she may uncover is the same as that which she may uncover in      front of other women, so she may uncover her head, neck, feet, hands,      forearms and calves, etc. But she should not make her clothes short. 
          Majmoo’ al-Fataawa, 12/276, 277 
     See the answers to      questions no, 12371,      6569 for more quotations and explanations from the      scholars concerning what a woman may and may not show in front of other      women and her mahrams. 
           Secondly: 
          Some scholars differentiate between Muslim women and kaafir      women, and do not allow taking off the hijab in front of kaafir women. This      is a less correct view, because Jewish women used to enter upon ‘Aa’ishah      and others among the women of the Sahaabah, and it is not known that they      wore hijab in front of them. 
          The scholars of the Standing Committee were asked: 
          Is it obligatory to wear hijab in front of kaafir women, or      can we interact with them as we interact with Muslim women? 
          They replied: 
          There are two views among the scholars regarding this matter.      The more correct view is that it is not obligatory, because no such thing      was narrated from the wives of the Prophet  (peace and blessings of      Allaah be upon him) or from others among the women of the Sahaabah when they      met with Jewish and idolater women in Madeenah. If that had happened it      would have been transmitted as less serious matters were transmitted.   
          And they said: 
          There is no reason why a woman should not uncover her face in      front of another woman, whether she is a Muslim or a kaafir, because women      are only commanded to cover their faces in front of men who are not their      mahrams. Allaah says (interpretation of the meaning): 
     “…and to draw their veils      all over Juyoobihinna (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 women…”
          [al-Noor 24:31] 
          So Allaah commanded them to draw their veils over their faces      in front of men, except for the mahrams mentioned in this verse, or those      who are their mahrams because of breastfeeding (radaa’ah), as mentioned in      other verses. 
          What is meant by “women” in this verse is all women, both      Muslim and non-Muslim. And Allaah knows best. 
          Fataawa al-Lajnah al-Daa’imah,      17/287, 288 
          And Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have      mercy on him) was asked: 
     Is it permissible for a      Muslim woman to uncover her hair in front of a non-Muslim woman, especially      as she may describe the Muslim women to her male relatives who are not      Muslim? 
          He replied: 
          This is a matter that is based on different scholarly      interpretations of the verse in which Allaah says (interpretation of the      meaning): 
     “And tell the believing      women to lower their gaze (from looking at forbidden things), and protect      their private parts (from illegal sexual acts) 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 Juyoobihinna      (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 women…”
          [al-Noor 24:31] 
          The scholars differed concerning the pronoun in the word      nisaa’ihinna (“their women”). Some of them said that what is meant is      the gender, i.e., all women in general. And some of them said that what is      meant is the type of women, i.e., believing women only. According to the      first view, it is permissible for a woman to uncover her hair and face in      front of non-Muslim women, but according to the second view it is not      permissible. We are more inclined towards the first view because it is more      sound, because among women there is no difference between a Muslim woman and      a non-Muslim woman. This is so long as there is no fitnah (temptation). But      if there is the fear of fitnah, such as if the woman may describe her (the      Muslim woman) to her male relatives, then it is essential to take      precautions in that case, and the woman should not uncover any part of her      body, such as the feet or hair, in front of the other woman, whether she is      Muslim or non-Muslim. 
          Fataawa al-Mar’ah al-Muslimah –      compiled by Salaah al-Deen Mahmoud, p. 605. 
          And Allaah knows best.