Answer
Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
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.
Answering your question, the Fatwa Center at Islam Q and A, states:
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 fetus whose mother is beaten to death, so that fetus 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.
Allah Almighty knows best.
Editor’s note: This fatwa is from Ask the Scholar’s archive and was originally published at an earlier date.