Is it permissible for one of Fatimah's sons to marry Umm Kulthum, knowing that Aminah (the grandmother) breastfed Fatimah, and then Aminah breastfed Umm Kulthum?

If Aminah breastfeeds Fatimah and Umm Kulthum, they become foster sisters. Consequently, Fatimah's children are not permitted to marry Umm Kulthum because she is their maternal aunt through fosterage. The Prophet, peace and blessings be upon him, said: "What is prohibited by lineage is also prohibited by fosterage." Just as a maternal aunt by lineage is prohibited (for marriage), so too is a maternal aunt by fosterage. This is what Ibn Qudamah stated. The prohibition due to fosterage depends on five distinct acts of suckling before the child reaches the age of two years.