Is it permissible for a child to be suckled by his maternal grandmother, thereby becoming a maternal uncle to his female cousins (daughters of his aunts) and a paternal uncle to his female cousins (daughters of his uncles)? And is there a reference that clarifies this issue?

If an infant breastfeeds from his maternal grandmother five full, satisfying times within the period of breastfeeding, his grandmother becomes his mother through fosterage, and his maternal uncles and aunts become his siblings. Consequently, it becomes unlawful for him to marry his maternal aunts' daughters and his maternal uncles' daughters, due to the Prophet Muhammad's (peace be upon him) saying: "What is unlawful by lineage is unlawful by fosterage."

For more information on the prohibitions of fosterage, one can refer to books of Islamic jurisprudence, such as "Fiqh As-Sunnah (definition)" by Sayyid Sabiq, or the interpretation of the verse concerning prohibited women in Surah (definition) An-Nisa (4:23) from Al-Qurtubi's tafsir (definition), "Al-Jami' li Ahkam Al-Quran."