Her husband’s father does not pray because he denies that it is obligatory; how should she interact with him?
Question

                I have received a marriage proposal from a religious man whose religious commitment and character are pleasing to me, buat his father does not pray because he denies that it is obligatory. But the problem is the family living arrangements, i.e., I would be living in the family home with his father. So how should I interact with him? Are there any guidelines on interacting with him, or would he be like any other mahram except that he does not pray? In other words: is it permissible to eat and drink with him and to uncover in front of him? Please note that I wear niqab.
Praise be to Allah.Firstly: 
    
    The father mentioned in the question, who 
    does not pray because he denies that it is obligatory, is a kaafir 
    (disbeliever) in the sense of major kufr that puts him beyond the pale of 
    Islam; there is no difference of opinion concerning that among any of the 
    scholars. Such a thing can almost never be done except by stubborn atheists, 
    such as communists and secularists. Whoever denies anything of that which 
    was brought by the Messenger (blessings and peace of Allah be upon him) 
    becomes a disbeliever thereby. If the matter has to do with prayer and other 
    pillars of Islam and its major fundamentals, then that is even worse, 
    because there is no excuse for the one who denies and rejects it, as it is 
    so well known and people grow up in Muslim countries knowing that and 
    respecting it, even those who do not do it. Hence no one would dare to deny 
    that it is obligatory and reject it except one who has gone to extremes in 
    disbelief and stubbornness – Allah forbid. 

    Shaykh al-Islam (Ibn Taymiyah – may Allah 
    have mercy on him) said: 

    The sin of the apostates is worse before 
    Allah, and in the eyes of His Messenger and the believers, than one who is 
    originally a disbeliever, in several ways. These people are definitely to be 
    executed so long as they do not come back to that which they have left; it 
    is not permissible to make any treaty or peace deal with them or to offer 
    them safety, or to free their prisoners or ransom them in exchange for money 
    or in a prisoner exchange; it is not permissible to eat meat slaughtered by 
    them, or to marry their women or to take them as slaves so long as they 
    remain apostates, according to scholarly consensus. Both their fighters and 
    those who are not fighters – such as old men, the blind and the chronically 
    ill – are to be executed, according to scholarly consensus; that also 
    applies to their women, according to the majority of scholars. 

    In the case of one who is originally a 
    disbeliever, it is permissible to enter into treaties with him and grant him 
    security; it is permissible to show kindness to him and to ransom him if he 
    is a prisoner, according to the majority of scholars. If he is one of the 
    People of the Book, it is permissible to offer them protection under Muslim 
    rule, and to eat their food and marry their women, and their women are not 
    to be fought and killed unless they take part in the fighting in word or in 
    deed, according to scholarly consensus. Similarly, none of them are to be 
    executed except those who are fighters, according to the majority of 
    scholars, as is indicated by the Sunnah. The disbeliever who is an apostate 
    is worse in terms of religion and worldly standing than the disbeliever who 
    remains a disbeliever. 

    End quote. Majmoo‘ al-Fataawa 
    (28/413-414) 

    Secondly: 

    If the father is as you described him, then 
    we do not advise you to agree to marry his son, unless he will provide you 
    with separate accommodation that is far away from this father. In fact that 
    is the basic and natural requirement: that you should get married and have 
    your own accommodation, separate from his family and your family. 

    The fact that the husband is staying in the 
    family home and living with this father as if nothing happened, and as if he 
    has not committed this grave action (of denying and forsaking prayer), 
    cannot be overlooked. Rather he should exhort him to fear Allah and rebuke 
    him for his disbelief and misguidance. Then if he persists in it, he should 
    leave him and avoid mixing with him, and he should protect his wife and 
    children from him, so long as he remains an apostate. 

    Please see also the answer to question no.
    141680 

    And Allah knows best.