Is it permissible to marry my female cousin if her maternal aunt (i.e., my second paternal aunt) breastfed both of us together?

If your paternal aunt breastfed the questioner and his paternal aunt's daughter five full feedings, then his paternal aunt's daughter is his sister through rada'a (nursing) and it is impermissible for him to marry her. If the nursing was less than five feedings, then prohibition is not established thereby, and it is permissible for him to marry her, unless she is prohibited to him from another aspect.