What can a Muslim woman uncover in front of kaafir mahrams and women?

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.