Is it permissible for me to marry my female cousin's son's sister, who was nursed with my brother?

Your brother's suckling from your paternal aunt's son's mother does not establish a prohibition (of marriage) between you and your paternal aunt's daughter. This is because the suckling of one sibling from a woman who is not their own mother does not affect the remaining siblings; rather, its ruling is restricted solely to the suckling infant. Therefore, it is permissible for you to marry this daughter of your paternal aunt.