Is my father's paternal cousin's daughter, whom I breastfed with, considered my sister, and are my father, mother, and siblings considered her mahrams or relatives in Islamic law?

If your paternal uncle's daughter was suckled by your father's milk, she is considered his daughter and a sister to all his children. However, if she was suckled with you by a foreign woman, she is a foreigner to him and a sister to whoever was suckled with her. The nursing mother is a mother in every case, and her children are siblings to those whom she suckled.