Inheritance in the Islamic Sharee‘ah (legislation) has three causes:
First cause: Marriage
Under Sharee‘ah, marriage refers to the valid marital contract -- even in the absence of meeting in seclusion between the spouses or sexual intercourse between them. Both spouses inherit from each other, so long as the marital contract is in effect between them. For example, if a man divorces his wife and her ‘Iddah (post-marriage waiting period) elapses, then there would be no mutual inheritance between them, due to the termination of the marriage, which is the cause of inheritance.
However, if he divorces her during his fatal sickness and is accused of having done so with the intention of preventing her from inheritance, in this case, she inherits (her share of) his property even (if he dies) after the conclusion of her ‘Iddah, and even if she gets married to another person according to the opinion of some scholars of Fiqh (Islamic jurisprudence), by way of opposing the wrongful husband and nullifying his purpose.
Second cause: Walaa’ (allegiance of an ex-slave to his ex-master)
This is a kind of kinship. If a master frees his slave, a kind of relation called Walaa’ (allegiance) starts to exist between them. It is a favor done by the master to his slave out of his bounty, whereby he brings him out of slavery to freedom.
Third cause: Real Kinship
Kinship is a connection between two persons by virtue of birth, be it a close or far connection. Every man or woman with whom one has a connection by birth, no matter how close or far, whether from the side of the father or from the side of the mother, or from the side of both together, is one's kin. This is the strongest cause of inheritance.
The heirs from among the kith and kin are divided into three divisions:
1- The ascendants: The deceased's father, grandfather (the father’s father), up to all levels of similar relationships from the side of the father; and the deceased's mother, and grandmother being related to him/her through a male or a female heir.
2- The descendants: The deceased's sons and sons of his son down to all levels, and his son's daughter down to all levels of her father.
3- Non-ascendant /descendant relations: They are the deceased's siblings;whether they are full or half-brothers or sisters, the male children of his/her full brothers and half-brothers from the side of the father, his paternal uncles who are his father's full brothers, or half-brothers from the side of the father up to all levels, the paternal cousins down to all levels.
These are the three causes of inheritance agreed upon among scholars. They deserve, in the presence of a sound mind and disposition, to be causes of inheritance. There are other causes which are subject to dispute, as to whether or not they are regarded as causes of inheritance, like Bayt Al-Maal of the Muslims (Muslim treasury), the kith and kin who are not heirs, etc.