Is it permissible for me to marry my maternal aunt’s daughter, who was nursed with my brother by her mother, knowing that my mother did not nurse her? And is she considered my foster sister or only my brother’s foster sister?

What is prohibited by lineage is also prohibited by fosterage. Therefore, whoever is nursed by a woman becomes her child and the child of her husband, the owner of the milk. This applies specifically to the nursed individual, not to his brothers and sisters. Consequently, your aunt nursing your brother does not include you, and it is permissible for you to marry your aunt's daughter, unless there is proof of a lawful fosterage between you from another source, bearing in mind that the fosterage that establishes prohibition must consist of five distinct nursings.