Does the deceased grandmother inherit after her deceased son (the father), does she have a share in his estate, and to whom does this share devolve after her death?

The deceased's debt must be paid from their estate before its distribution, as debt takes precedence over the heirs' right. If a husband and wife die together and it is unknown which of them died first, neither inherits from the other. The father's estate is divided among his heirs, and the mother's estate is divided among her heirs. If your grandmother died after your father, she has a share in his estate that transfers to her heirs.

The mother of the deceased receives one-sixth due to the presence of a lineal descendant (heir), and the remainder goes to the sons and daughters by ta'sib (residuary heirship), with the male receiving the share of two females. The brothers and sisters receive nothing as they are excluded by the son by complete exclusion (hajb hirmān). The estate is divided into twelve shares: the mother receives two shares, each son receives two shares, and each daughter receives one share.