Is it religiously permissible for a young man to marry his maternal uncle's daughter, given that the young man's paternal aunt breastfed him, knowing that the young man's paternal aunt is the maternal uncle's sister from his mother's side, and also his paternal grandfather's wife? And what is the nature of the kinship between the young man and his maternal uncle?

If the suckling occurred five times, satiatingly, within the two years, and the milk for the suckling was due to your grandfather's intercourse, then this marriage is null and void, and you must separate from her immediately. This is because the woman is one of your mahram (definition) (unmarriageable kin); your paternal aunt has become your mother through suckling, your grandfather your father, your maternal uncle your brother, and his daughter your brother's daughter.