Is it permissible to marry my female cousin if my mother breastfed her along with my sister, and if my elder brother was breastfed by my aunt?

What is established by lineage is also established by fosterage. If your female cousin suckled from your mother five full sucklings before the age of two, then she becomes your foster-sister, and it is forbidden for you to marry her. The fact that she did not suckle with you does not prevent the prohibition from being established. However, if the suckling was less than five sucklings, then the prohibition is not established. As for your elder brother suckling from your paternal aunt, it does not affect the permissibility of marriage between you and her.