Is it permissible to distribute the estate of a person who has lost his memory or is terminally ill?
Question

                Is it permissible to divide the wealth of a man who has Alzheimer's (extreme weakness in the memory and understanding) among his heirs? What is the Islamic ruling in this case?.
Praise be to Allah.
    It is not permissible for 
    anyone to inherit from one who is still known to be alive, even if he is 
    sick and has lost his memory, or is no longer accountable because of old age 
    or senility or loss of memory or physical weakness. Rather this is the case 
    even with regard to terminal illness, because one of the conditions of 
    inheritance that are agreed upon among the scholars is that the person 
    should have died, either in a real sense or according to a legal ruling, as 
    in the case of a person who is missing and of whom no news is heard, or by 
    assumption, as in the case of a foetus whose mother is beaten to death, so 
    that foetus inherits from the blood money on the assumption that he was 
    alive [when his mother died], then it is assumed that he died so that his 
    share of the blood money can be inherited from him. 

    In al-Mawsoo’ah 
    al-Fiqhiyyah (2/33) it says: 

    There are three conditions 
    for inheritance: 

    1.That 
    the death of the person be established, or he be regarded as dead on the 
    basis of a ruling, such as a missing person if the judge rules that he has 
    died, or on the basis of assumption, as in the case of the foetus who was 
    separated from his mother by a criminal act, which requires giving a slave 
    as diyah or blood money.

    2.That 
    the heir be proven to be alive in a real sense after the death of the 
    person, or on the basis of assumption, like a foetus who came out alive in a 
    stable manner for a time that shows that he was alive when the mother died, 
    even if it (the foetus) was very small.

    3.That 
    it be known who is entitled to the inheritance, such as spouses, relatives 
    or freed slaves; how they are related (to the deceased), such as whether 
    they are children, fathers, mothers, siblings or paternal uncles; and the 
    degree of relationship that connects the deceased to the heir.

    End quote. 

    Shaykh Muhammad ibn Saalih 
    al-‘Uthaymeen (may Allaah have mercy on him) said: 

    As for the fact that the one 
    whose estate is to be inherited should have died, that is because of the 
    words of Allaah (interpretation of the meaning): “If it is a man that 
    dies leaving a sister, but no child, she shall have half the inheritance” 
    [al-Nisa’ 4:176]. What is referred to here is death, and leaving behind 
    his wealth cannot happen except by his moving from this world to the 
    Hereafter. 

    Death is ascertained by 
    observation, or when it becomes common knowledge, or when two witnesses of 
    good character testify to that. 

    With regard to death on the 
    basis of a legal ruling, this applies in the case of one who has gone 
    missing, if the period determined for looking for him has ended; in that 
    case the rule is that he has died on the basis of what may be the case, 
    meaning that the possibility that he has died is to be regarded as if it is 
    certain when it is not possible to be certain, because of the actions of the 
    Sahaabah (may Allaah be pleased with them). 

    Tasheel al-Faraa’id, 
    p. 18, 19 

    Based on that: 

    It is not permissible to 
    divide the wealth of a man who has been affected with Alzheimer's or any 
    other kind of sickness, so long as he is still alive. 

    Moreover, we should point out 
    two related matters: 

    1.It is 
    possible that Allaah may decree healing for this sick person.

    2.It is 
    possible that one of the heirs may die before the sick person does, and if 
    one of the heirs dies before the person from whom he would inherit, then his 
    share of the estate dies with him; if he has taken his inheritance from that 
    person whilst the latter is still alive, then he will have taken wealth that 
    is not permissible for him.

    And Allaah knows best.