Is it Islamically permissible for a girl to marry her female cousin's son, knowing that the girl was breastfed by her aunt who was three months pregnant, and that the girl's mother breastfed her female cousin's eldest daughter?

If your paternal aunt breastfed you five full feedings within the first two years (of your life), then all her children become your siblings through suckling, whether they were breastfed with you or not. Accordingly, it is not permissible for you to marry this young man because he is your brother through suckling. It makes no difference whether he is the son of her first husband or her second, and your aunt's pregnancy at the time she breastfed you does not affect the ruling.