No, a girl cannot marry her mother's brother in Islam.
Her mother's brother is considered her maternal uncle (khaal), and marriage to a maternal uncle is explicitly prohibited in Islamic law. This prohibition stems from the familial relationship and the respect due to close relatives. The uncle-niece relationship creates a bond of kinship that precludes marriage.
Therefore, marrying one's maternal uncle (mother's brother), whether a full or half-brother, is forbidden (haram) according to Islamic principles.
