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

I was wondering is it permissible for me to mix, sit with and take off my hijab in front of my father-in-law who is kaafir? and how about my mother-in-law can i take my hijab off in front of her? can u also advice me how i should treat them?.
Praise be to Allah.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. 
Fatawa 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-Fatawa, 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 Allah 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. Allah 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 Allah 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 Allah knows best. 
Fatawa al-Lajnah al-Daa’imah, 17/287, 288 
And Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allah 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 Allah 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. 
Fatawa al-Mar’ah al-Muslimah – compiled by Salaah al-Deen Mahmoud, p. 605. 
And Allah knows best.