Is the paternal aunt's daughter considered a sister to my brother through rada'ah (milk-kinship), and thus a sister to the questioner, if she suckled from the questioner's mother more than five times?

Lactation establishes the same prohibitions as lineage, for the Prophet, peace and blessings be upon him, said: "What is prohibited by lineage is prohibited by lactation." So, whoever suckles from a woman becomes a son to her and to her husband, the owner of the milk, and a brother to all her children. Therefore, if your maternal aunt's daughter suckled from your mother five full and satisfying breastfeeds or more, she has become your sister through lactation.