Is it permissible for the nephew who was suckled by the mother to marry the sister, and what is the ruling on the marriage of the sons of the nephew who was suckled by the mother to the daughters of the brothers and sisters?

If your paternal uncle was breastfed by your mother five times, then he becomes a son to your mother through nursing and a brother to her children. Consequently, it is not permissible for your cousin (son of your paternal uncle) to marry your sister because she would become his paternal aunt through nursing. This is based on the Almighty's saying: "Forbidden to you are your mothers, and your daughters, and your sisters, and your paternal aunts, and your maternal aunts," and the Prophet's (peace be upon him) saying: "What is forbidden by lineage is also forbidden by breastfeeding."