Are the female cousins of the maternal aunt's daughter who were not breastfed by the mother considered sisters to the one who was breastfed, or are they non-mahram? And is the maternal aunt's son considered a brother to the sister of the one who was breastfed, due to his sister being breastfed with her?

Suckling prohibits what lineage prohibits. A child who suckles from a woman five times or more within the first two years becomes her son and the son of her husband, the owner of the milk. This prohibition applies to the suckling child alone, not to his siblings. Based on this, your female cousin who suckled from your mother has become a sister to you and your siblings through suckling. However, her sisters who did not suckle from your mother are not considered your sisters through suckling. And your male cousin is not a brother to your sister through suckling merely because his sister suckled from your mother; the suckling pertains only to his sister.