Answered by Sayyidi Habib Umar bin Hafiz (may Allah protect him and benefit us by him)
If someone’s mother is from Ahl al-Bayt is he or she considered to be from Ahl al-Bayt?
No, they are not legally considered to be from Ahl al-Bayt. Thus it is not forbidden for them to receive zakat and they do not receive a portion from the spoils of battle (khums). However, they are regarded to be among the relatives of the Ahl al-Bayt and among those that the Prophet ﷺ referred to be as being related to him through marriage.
The general principle is that although a person’s ties to his mother’s parents and relatives must be honoured, his lineage goes through his father and he is regarded to be a member of his father’s family or tribe. There are two exceptions to this. One is Sayyiduna `Isa, who had no father and is thus known by his mother – `Isa bin Maryam. In the Qur’an, Allah mentions Sayyiduna `Isa as being among the progeny of Sayyiduna Ibrahim, despite the fact that Sayyiduna `Isa is only descended from Sayyiduna Ibrahim through his mother, Sayyidatuna Maryam.
The other exception is the descendants of Sayyidatuna Fatimah, all of whom are considered to be the direct descendants of the Prophet ﷺ even though they are descended from him through their mother and not their father. This is why the Prophet ﷺ referred to Sayyiduna al-Hasan as his son. This is one of the Prophet’s unique attributes (khasa’is).