What is the ruling on my marriage to my maternal aunt’s daughter, and what is meant by the suckling that prohibits marriage, in the event that my mother says my aunt breastfed me, while my aunt now denies having breastfed me, despite there being evidence to support my mother’s statement?

If it is confirmed that you were suckled by your maternal aunt five times within the first two years of your life, then it is impermissible for you to marry her daughter, as she would be your sister's daughter through suckling. If the suckling is not confirmed with certainty, then the default is its absence, and it is not established by doubt. Nevertheless, if the suckling was widespread and known within the family, we do not advise you to marry in order to avoid suspicion. As for your brother being suckled by your grandmother, it is impermissible for him to marry her granddaughter (his maternal aunt's daughter) because he would become a brother to her daughters through suckling and a maternal uncle to his maternal aunts' daughters, due to the Almighty's saying: (Prohibited to you are your mothers, and your daughters, and your sisters, and your paternal aunts, and your maternal aunts, and the daughters of your brother, and the daughters of your sister), and due to the Prophet's (peace be upon him) saying: "Suckling prohibits what lineage prohibits."