Is it permissible for me to marry my brother's foster sister, knowing that my mother did not breastfeed her?

"Suckling establishes the same prohibitions as lineage. Whoever suckles from a woman becomes her child and the child of her husband, the owner of the milk. This applies exclusively to the suckled individual, not to their siblings.

Based on this, if your female cousin did not suckle from your mother, and there is no other established suckling relationship between you two, then it is permissible for you to marry her. Her brother having suckled from your mother does not make her unlawful for you, because that suckling is specific to him alone, not to his brothers or sisters."