A Muslim cannot Inherit From a Kafir
Although, the distribution of inheritance is based on nearness of kinship, but there are certain exceptions to this rule. First of all, the deceased and his inheritor should not be from two different religions. Therefore, a Muslim will not inherit from any kafir and no kafir from a Muslim, no matter what lineal relationship they may have between them. The Prophet sallallahu aalyhi wa sallam said, “The Muslim does not inherit the kafir, nor does the kafir (inherit) the Muslim.” [Mishkat, p. 263]
This rule relates to a situation when a person is a Muslim or a kafir by birth. But, if a person who was first a Muslim, turned away from Islam and became an apostate and died or was killed in that state, all his earnings while being a Muslim shall go to his Muslim inheritors, and whatever he may have earned after his apostasy shall be deposited in the Bayt al-maal (Public Exchequer).
But, if a woman becomes an apostate, all her property, whether acquired during her days of Islam or during apostasy, shall go to her Muslim inheritors. However, an apostate as such, man or woman, shall not inherit from any Muslim nor from any apostate.
The Inheritance of the Murderer
If someone kills a person from whose property he was entitled to receive a share, he shall no longer remain his inheritor and shall be excluded from the inheritance of the person whom he has killed.
The Prophet said, “The killer shall not inherit.” [Mishkat, p. 263]. However, some forms of qatl al-khata (accidental or unintended homicide) are exempted from this rule, details of which appear in books of Fiqh.
The Inheritance of the Unborn Child
If a person leaves some children and his wife is pregnant, then this unborn child will also be counted among inheritors. But, since it is either difficult or uncertain to determine the sex or the number of children in the mother’s womb, it would be appropriate to postpone the distribution of inheritance until the birth of the child. If, the distribution of property has to be made necessarily, then, as an expedient measure, one must suppose two situations in terms of a boy or a girl and distribute to the inheritors the lesser portion coming out of the two situations. The remaining should be held for the child-to-be-born.
(Taken from Maaruf-ul-Qur’an by Mufti Muhammad Shafi Usmani)