Is my parents’ marriage considered null and void, and does my mother become a sister to my father, given that my mother was breastfed by my grandmother (my father’s mother) more than five times, and are we, consequently, still attributed to our father?

If your mother was suckled by your father's mother five times within the two years of nursing before weaning, then she is considered their daughter and your father's sister through suckling. Prohibition (of marriage) and the state of being unmarriageable (mahramiyya) are established between them, and they must be separated without dispute among the imams. Their children are legitimately attributed to them, because the intercourse with their mother was a doubtful intercourse, in which the child is attributed to the father and inherits from him. It is advisable to refer to the Sharia (definition) judiciary to confirm the establishment of suckling.