He was breastfed by his maternal grandmother, along with his maternal cousin; what is the ruling?
Praise be to Allah.
In the answer to question no. 146850 we stated that the rulings on breastfeeding apply only to the one who was breastfed and his descendants. As for his siblings and the like, the ruling on breastfeeding does not include them. Based on that, you are not a brother through breast feeding to the sisters of your maternal cousin S.
The fact that you were breastfed by your grandmother makes you a brother through breast feeding to your mother, and hence to your maternal aunt. Based on that, you are a maternal uncle through breast feeding to all the daughters of your maternal aunts, whether or not they were breastfed by your grandmother. Thus you are a mahram to all of them on this basis.
But the ruling is more emphasized with regard to the one who was breastfed by your grandmother and thus became a sister to you through breast feeding, because you were both nursed at the same breast.
Shaykh Ibn Baaz (may Allah have mercy on him) said: If the child was breastfed by his grandmother five or more times within the first two years of life, he thus becomes a brother to his maternal uncles and aunts, a paternal uncle to the children of his maternal uncles and a maternal uncle to the children of his maternal aunts. Hence it is not permissible for him to marry any of the daughters of his maternal uncles, because he has become a paternal uncle to them through breast feeding, or any of the daughters of his maternal aunts, because he has become a maternal uncle to them through breast feeding, or any of their offspring.
End quote from Majmoo‘ Fataawa Ibn Baaz, 22/306
And Allah knows best.