Do the questioner and the older maternal cousin become siblings through رضاعة (nursing/wet-nursing), if the maternal aunt nursed the questioner's brother, and the questioner's mother nursed the maternal cousin's twin?

Your mother's breastfeeding of your cousin does not make him your brother through fosterage, except for him alone, not the rest of his siblings. The prohibitive breastfeeding must occur within the first two years of life and consist of five full, satisfying breastfeeds.