Is it permissible for my sons or daughters to marry the sons or daughters of my sister's daughter, who is my sister through rada'a (nursing)?

If your sister breastfed you five times while you were still within the first two years of life, then she becomes your mother through breastfeeding, and her children become your siblings, and you become their maternal uncle. It is not permissible for your sons to marry your sister's daughters (who are their paternal aunts through breastfeeding). And if your sister breastfed her granddaughter, then that girl becomes your sister through breastfeeding and an aunt to your children, and it is not permissible for your sons to marry her. However, it is permissible for them to marry her daughters and sons. And it is permissible for your sons to marry the sisters of this girl if they were not breastfed with you. The fundamental principle for this is the saying of the Prophet, peace and blessings be upon him: "What is forbidden by lineage is also forbidden by breastfeeding."