foster sister
Refers to a girl who is not related by blood but was nursed by the same woman, making her a foster sister. The term is relatively rare and may be encountered in historical or literary contexts.
See also: 乳兄弟 (ちきょうだい)
She is my foster sister, raised by the same wet nurse.
Compound of 乳 (chi, 'milk') and 姉妹 (kyōdai, 'sisters'), literally 'milk sisters'. The term originates from the historical practice of wet-nursing, where children nursed by the same woman were considered foster siblings.