If the situation is as you mentioned, that
your paternal grandfather married your maternal cousin; his daughters from your cousin are your father's sisters, or your paternal aunts. It is impermissible for you to marry any of them, according to Allah's saying,
Forbidden to you (for marriage) are: your mothers, your daughters, your sisters, your father’s sisters
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.