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.