Is it permissible to marry a young man who is the son of the maternal aunt of the female questioner's mother, and whose older brother was suckled by the female questioner's mother?

You are permitted to marry the aforementioned young man if it is not proven that he was suckled by your mother or from any other source. His brother being suckled by your mother does not make him a forbidden (mahram) relative to you; rather, it only makes his brother your maternal uncle through suckling. The suckling that establishes prohibition (mahramiyyah) consists of five satisfying sucklings.