Rules regarding will (Wasiyyat) » Inheritance

2737. There are three groups of persons who inherit from a dead person, on the basis of relationship:

1. The first group consists of the dead person's parents and children, and in the absence of children, the grand children, however low, and among them whoever is nearer to the dead person inherits his property. And as long as even a single person from this group is present, people belonging to the second group do not inherit.
2. The second group consists of paternal grandfather, paternal grandmother, and sisters, brothers, and in the absence of sisters and brothers their children, whoever from among them is nearer to the dead person, will inherit from him. And as long as even one person from this group is present, people belonging to the third group do not inherit.
3. The third group consists of paternal uncles and paternal aunts and maternal uncles and maternal aunts, and their descendants. And as long as even one person from the paternal uncles and paternal aunts and maternal uncles and maternal aunts of the dead person is present, their children do not inherit.
   However, if the paternal step uncle and the son of the real paternal uncle are present, the son of the dead person's real paternal uncle will inherit from him to the exclusion of the paternal step uncle. But if there are several paternal uncles and several paternal cousins, or if the widow is alive, then this rule is not without Ishkal.

2738. If the dead person's own paternal uncle and paternal aunt and maternal uncle and maternal aunt and their children and their grandchildren do not exist, the property will be inherited by the paternal uncles and paternal aunts and maternal uncles and maternal aunts of dead person's parents.
And if even they do not exist, the property will be inherited by their descendants. And in the absence of their descendants, the property is inherited by the paternal uncles and paternal aunts and maternal uncles and maternal aunts of the dead person's paternal grand parents. And if even they do not exist, the property is inherited by their descendants.
2739. Husband and wife inherit from each other as will be explained later.
(Islamic Laws: Ayatullah al Sistani)