I'm currently engaged to one of my aunt's (mother's sister) daughters. My fiancé has a brother who was breastfed from another one of my aunts (another of mother's sister) more than five times. This aunt had a child a couple of months later and of course breastfed her son, and my fiancé's brother. I have never breastfed from anyone of my aunts. First, I would like to know if it is permissible for me to marry her. Second, who is considered Mahram to my fiancé (i.e. the cousin who breastfed with her brother at the same time).

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Since you were not breastfed with this girl [by your aunt] and she was not breastfed by your mother and there are no marriage impediments like her being in a waiting period after being divorced from another man, etc., then there is no harm on you to marry her.

As regards your cousin from other aunt who was breastfed from the same woman as the brother of this girl, he does not become a Mahram (a non-marriageable relative) to her just because of this breastfeeding.

Allaah knows best.