Do the sisters of my female cousin – whom my grandmother breastfed five times – become unlawful for me (to marry)?

Your aunt's daughter, who was suckled by your grandmother, is considered your grandmother's daughter through suckling (rada'a), and thus she is one of your mahram (unmarriageable kin). As for her sisters who were not suckled by your grandmother, they are not forbidden to you, unless they are your mahram from another relation. Their elder sister having been suckled by your grandmother does not make them forbidden. The suckling that is legally binding for prohibition (tahrim) is five sucklings, according to the more preponderant opinion.