If he lets one of his sons live in an apartment for free, is he obliged to treat the rest of his sons equally?
Question

                I have two daughters and four sons. One of the daughters is married and one of the sons is also married. I built four apartments for the sons to live in for free, on condition that they give the girls their right from the rents, on the basis that each male will have the share of two females. Now my son who is married is living in one of these apartments and he is giving one third of the rent to his married sister only. But does he have to give something to his sister who is not married, and who is under my care at present? Similarly does he have to give to his brothers who are also under my care at present? Please note that they will get similar apartments after they marry, if Allaah wills.
Praise be to Allah.
    Firstly: 

    It is essential to differentiate between maintenance and 
    gifts. The maintenance that the father spends on his children varies 
    according to their needs. What is spent on a small child is not the same as 
    what is spent on an older child, and the expenses of a child who is studying 
    at the elementary stages is not like the expenses of a child who is studying 
    at the University. The expenses of a daughter are not usually like the 
    expenses of a son. 

    This includes the expenses of marriage. If a father spends on 
    the marriage of one of his sons, he is not obliged to give something similar 
    to those who are not married, because this comes under the heading of 
    expenses which are given only to the one who needs it. When another one 
    grows up he will arrange his marriage too, and so on. 

    This may also be said to regard to accommodation. If the 
    father has a house or apartment, and the married son needs it, the father 
    may give it to him for him to make use of it, not to own it, and thus he 
    meets his need. The same applies if he needs a car for example; he may give 
    it to him to drive it but not to take possession of it. 

    The point is that he may give each son what he needs and what 
    is appropriate for him. 

    As for gifts which are additional to maintenance, this is 
    where equality and fairness is required and it is haraam to prefer one son 
    over the others. Sons should be given equal gifts, and each male is given a 
    share of two females.  

    Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said, 
    pointing out the difference between maintenance and giving: 

    if we assume that one of them is at school and needs expenses 
    for school such as books, exercise books, pens, ink and the like, and the 
    other one does not need that, and he is older than him but he does not need 
    those things, if he gives something to the first one does he have to give 
    something similar to the second? 

    The answer is that he does not have to do that, because being 
    fair in spending on maintenance means giving each one what he needs. An 
    example of that is if a son needs a ghutrah and cap which costs 100 riyals, 
    and the female needs earrings which cost 1000 riyals. What is fair? 

    The answer is that what is fair is to buy the headgear for 
    the boy for 100 riyals, and to buy earrings for the girl for 1000 riyals, 
    which is 10 times more than what the boy gets. This is fair. 

    Another example: if one of them needs to get married and the 
    other does not, what is fair? 

    The answer is that he should give the one who needs to get 
    married and not to the other. It is to be regarded as a mistake when some 
    people arrange marriages for their sons who have reached the age of 
    marriage, and they have younger sons, so they write in their wills that the 
    sons who are not yet married are to have marriages arranged for them with 
    the costs covered from one third of the will [which it is permitted to 
    bequeath]. This is not permissible, because marriage comes under the heading 
    of meeting needs, and these children have not reached the age of marriage, 
    so making this will in their favour is haraam and should not be executed, 
    because even the heirs are not permitted to execute the will except if they 
    are of the age of maturity and give their consent, in which case each heir 
    may decide to carry out these wishes with regard to his own share. End quote 
    from al-Sharh al-Mumti’ (4/599). 

    With regard to the obligation of treating children equally in 
    giving, please see the answer to question number
    22169. 

    Secondly: 

    On the basis of the above, if a person has a son who is 
    married and needs accommodation, there is nothing wrong with giving him 
    permission to live in one of the apartments, whilst retaining ownership of 
    it, and he does not have to give anything to his brothers or sisters. 
    Similarly, you do not have to give them anything, so long as they are not in 
    need of accommodation, either because they are living with you and under 
    your care, or because the daughter is living with her husband. 

    If the son does not need accommodation, then letting him live 
    in the apartment for free comes under the heading of giving, in which case 
    you have to pay attention to fairness, by giving the rest of his siblings 
    something equivalent to the rent of the apartment, or by making the son pay 
    rent which should go to you, or giving all of his siblings their share of 
    the rent. 

    If you want to give your children ownership of the 
    apartments, then you have to treat them fairly and give each male twice the 
    share of each female, as in the case of inheritance. It is wrong to give the 
    apartment to the son, for example, and then tell him: Give to your brothers 
    and sisters, or to put it in the ownership of the males and tell them to 
    give to the females, because they may or may not give to them. The fact that 
    the son needs the accommodation -- if he indeed needs it – can be met by 
    letting him live there, without giving him possession. If the father wants 
    to give him possession, then he must treat them all fairly, and it must be 
    documented properly in order to prevent disputes and differences between the 
    children. 

    May Allaah help us all to do that which He loves and which 
    pleases Him. 

    And Allaah knows best.