Is it permissible to marry Saad's daughter - Fathiya - knowing that the questioner was breastfed by his paternal grandfather's third wife - Kulthum - who is Saad's paternal grandmother?

The most correct opinion is that those who shared milk from the intercourse of one man are foster siblings, even if they were not nursed by the same mother. This is what is known as "fahl milk" (milk attributed to the father), through which prohibition is established. Therefore, the husband of the woman from whom you nursed is your foster father, and the prohibition extends to his descendants and ancestors. Thus, his children are your foster siblings, and your foster brother's daughter is forbidden to you, based on the Almighty's saying: ﴿and daughters of the brother﴾ and the Prophet's ﷺ saying: "What is forbidden by lineage is also forbidden by fosterage."