The origin for the women with whom marriage is prohibited, has to be, and is clearly mentioned, in the Kitab and Sunnah. As for the Kitab, Allah (swt) says: “And marry not women from whom your fathers married, except what has already passed; indeed it was shameful and most hateful, and an evil way .” [An- Nisa: 22] And He (swt) said:

“Forbidden to you (for marriage) are: your mothers, your daughters, your sisters, your father's sisters, your mother's sisters, your brother's daughters, your sister's daughters, your foster mother who gave you suck, your foster milk suckling sisters, your wives' mothers, your step-daughters under your guardianship, born of your wives to whom you have gone in - but there is no sin on you if you have not gone in them (to marry their daughters), the wives of your sons who (spring) from your own loins, and two sisters in wedlock at the same time, except for what has already passed; verily, Allah is Oft-Forgiving, Most Merciful. Also (forbidden are) women already married, except those (captives and slaves) whom your right hand possesses. Thus has Allah ordained for you. All others are lawful”. [An- Nisa: 23-24] As for the Sunnah, Abu Hurayra has reported that the Messenger of Allah said: “One should not bring together a woman and her father’s sister, nor a woman and her mother’s sister in marriage”. Muslim narrates that 'Aisha (ra) said that the Messenger of Allah (pbuh) said: “Suckling makes women unlawful (for marriage) the same as birth makes unlawful.” Thus, the marriage to mothers is absolutely forbidden. The mothers are everyone to whom you are related by childbirth. This is so, whether the one whom you are related to qualifies for the true word 'mother' which is the one who gave birth to you or she is a metaphorical mother i.e. is the one who gave birth to the one who gave birth to you and even if she is higher (in the line of descent)/and beyond. Therefore, your grandmothers: your mothers' mother and your fathers mother; the grandmothers of your mother and father; and the grandmothers of your grandmother and grandfather and even higher, irrespective of whether they are entitled to inheritance from you or not, they are all metaphorical mothers to whom marriage is prohibited.