Is it permissible to marry the daughter of my paternal cousin, knowing that my paternal grandmother breastfed both me and my paternal cousin, such that my paternal cousin is my sister through breastfeeding?

The daughter of a foster sister is forbidden (for marriage) just as the daughter of a blood sister is forbidden, due to the Prophet's (peace and blessings be upon him) saying: "What is forbidden by lineage is forbidden by fosterage." Therefore, if your paternal cousin is your foster sister, then it is not permissible for you to marry her daughter.