Only wives of Prophet, sallallaahu 'alayhi wa sallam, are mothers of believers
8-6-2016 | IslamWeb
Question:
In Pakistan and India, there are people known as Shahs; they say that these people belong to the family of Muhammad, sallallaahu 'alayhi wa sallam. They forbid marriage of their females outside of their families because they say that their females have the position of mother for the people outside their family. Please tell me whether this is correct according to Islamic teachings or not. As far as I know, the wives of Muhammad, sallallaahu 'alayhi wa sallam, were considered the mothers of other people. I do not know any other teaching about any other females of the family of Muhammad, sallallaahu 'alayhi wa sallam, because all daughters of Muhammad, sallallaahu 'alayhi wa sallam, were not married only inside the family of muhammad, sallallaahu 'alayhi wa sallam.
Answer:
All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad 
is His slave and Messenger.
The wives of the Prophet are the mothers of the believers by the text of the Quran, as Allaah says (what means): {…and his wives are [in the position of] their mothers.} [Quran 33:6]
The wives of the Prophet are the women whom Allaah forbade the Muslims from marrying, as Allaah says (what means): {And it is not [conceivable or lawful] for you to harm the Messenger of Allaah or to marry his wives after him, ever...} [Quran 33:53]
This ruling does not apply to other women, regardless of whether they are from the family of the Prophet or not.
The most preponderant opinion of the scholars is that eligibility in marriage is in regard to religion (religious commitment) only, and there is no hadeeth that proves that eligibility is in one's family lineage. This is the view of Imaam Maalik and the scholars who agreed with him. What supports this is the saying of Allaah (which means): {Indeed, the most noble of you in the sight of Allaah is the most righteous of you.} [Quran 49:13]
We have already clarified the ruling of marriage between the family of the Prophet and others in fataawa 102392 and 84626.
Also, for more benefit on belonging to the family of the Prophet please refer to fatwa 87526.
Allaah knows best.