She lives with her husband’s family even though there is a separate apartment available
Question

This is not my situation only, rather it is faced by many women who wear shar’i hijab (niqaab). We live with our husbands and their families in the same house, i.e. his brothers, who are men, live with us in the same house, and there is no servant, so we have to take care of the housework, which is plentiful and hard. There is nothing to stop one of the husband’s relatives, such as his paternal or maternal uncle, entering upon us without permission, so the house is open. When we clean the balconies, the neighbours and everyone in the street can see us. 
Is it correct for us to wear niqaab only when going out? Or should we wear it in the house from morning to night? Please note that this will be extremely difficult for us. I have a flat of my own, but we only go there to sleep. We live with my husband’s family that is composed of one brother and his mother. Please note that we can afford to live separately, but my husband does not want that. What should we do?.
Praise be to Allah.Hijaab is enjoined in all cases where it is thought that fitnah (temptation) may occur. For a large family to live together in one house, where the wife is together with her husband’s brothers or with his cousins (paternal or maternal) is a situation where the barrier of modesty may be lifted and shaytaan may instil evil ideas in people’s minds. In that case it is essential for a woman to cover her face in front of non-mahrams, and put up with any hardship involved. 
On this site we have a number of similar questions in which we have confirmed the obligation to wear hijab and cover oneself. Please see questions no. 6408, 13261, 40618, 47764 and 52814. 
Shaykh ‘Abd al-‘Azeez ibn Baz (may Allah have mercy on him) said: 
The husband’s brother, sister’s husband, cousins and so on are not mahrams, and they have no right to look at a woman’s face. It is not permissible for her to take off her jilbab in front of them, because that may lead to them being tempted by her. It was narrated from ‘Uqbah ibn ‘Aamir (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: “Beware of entering upon women.” An Ansaari man said: O Messenger of Allah, what do you think of the in-law? He said: “The in-law is death.” Agreed upon. What is meant by the in-law is the husband’s brother, uncle and so on. That is because they can enter the house without any suspicion, but they are not mahrams by mere virtue of their relationship to her husband. Based on that, it is not permissible for her to uncover her adornments in front of them, even if they are righteous and trustworthy, because Allah has limited the permissibility of showing one's adornments to mahrams only, and the husband’s brother, uncle, cousin and so on are not among the mahrams. The Prophet (peace and blessings of Allah be upon him) said in a hadeeth whose soundness is agreed upon: “No man should enter upon a woman except with a mahram.” What is meant by a mahram is one for whom it is permanently forbidden to marry her because of blood ties, ties of marriage or breastfeeding, such as a father, son, brother, paternal uncle, and so on. The Messenger of Allah (peace and blessings of Allah be upon him) forbade that lest the Shaytaan tempt them to commit evil and make sin attractive to them. It is proven that the Prophet (peace and blessings of Allah be upon him) said: “No man is alone with a woman but the shaytaan is the third one present.” Narrated by Imam Ahmad with a saheeh isnaad from ‘Umar ibn al-Khattaab (may Allah be pleased with him). 
Those who follow the customs of their countries that go against that on the grounds that this is their family’s tradition or the custom of their people have to strive hard to give up this custom. They should cooperate to put an end to it and rid themselves of its evil, so as to protect their honour and help one another in righteousness and piety, and in obedience to the command of Allah and His Messenger (peace and blessings of Allah be upon him). They should repent to Allah, may He be glorified and exalted, from what they have done in the past, and strive to enjoin what is good and forbid what is evil, and persist in doing so. They should not fear the blame of the blamers when supporting the truth and combating falsehood, and they should not let the fact that some people mock them or make fun of them put them off. What is required of every Muslim is to follow the laws of Allah willingly, seeking that which is with Allah and fearing His punishment, even if that means going against the closest and dearest of people to him. End quote. 
Fatawa al-Shaykh Ibn Baz (4/256-258) 
Our advice to the husband is to strive hard to provide accommodation that is separate from his family, and not to take such matters lightly. Allah has entrusted his wife to him and he must take care of her. Insisting on living in the same house as his family and brothers is not taking care of her. 
It is not one of the husband’s rights over his wife to force her to live with his family, rather he is obliged to provide her with separate accommodation when she asks for that. This has been discussed in detail, with evidence and the comments of the scholars, in the answer to question no. 7653. 
Situations such as that which you describe are not free of evils. It is very hard for the woman to wear full hijab – even in the house – from morning till night. She cannot do her housework when she is wearing full hijab; she has to uncover her face and hands, and so on, and in that case she will be seen by her husband’s brothers and others for whom it is not permissible to see her and in front of whom it is not permissible for her to uncover. 
It is not permissible for the husband to impose this on his wife, as it will gradually weaken her faith and lessen her modesty. He is a shepherd and is responsible for his flock. 
You say in your question that you have a separate flat. What is keeping your husband from treating you properly and looking after you in your own flat, instead of making you stay with your husband’s family all day even though that is difficult for you and leads to that which Allah has forbidden? 
We ask Allah to guide your husband to that which is best for you both in this world and in the Hereafter. 
And Allah knows best.