What is the ruling on a son acting as the guardian for his mother?

Question:
He says: Is it permissible for me to be the guardian for my mother in the case where it is not permissible for a son to be the guardian for his mother?

Answer:
Yes, the son is the guardian for his mother. The guardian for a woman is her father, or her grandfather, or her son, or her son's son, or her full brother, or her brother on the father's side. As for the brother on the mother's side, he is not a guardian. The brother on the mother's side is not a guardian for her. Her paternal uncle, her father's brother, her uncle, her father's brother, or her cousin.
That is, the rest of the 'asabah (residuary male heirs). The rest of the 'asabah are her guardians. As for her maternal grandfather, or her maternal uncle, or her brother on the mother's side, these are among the relatives by blood (dhawi al-aram) and they have no guardianship over her. Yes. And if no guardian from her 'asabah is found, then the Shari'ah court takes charge of the contract for her. Yes.