noun
foster sibling; milk sibling
Refers to a child who was nursed by the same woman as another child, creating a sibling-like bond. Not a legal or biological relationship. The term is gender-neutral; 乳兄 (ちあに) and 乳姉 (ちあね) can specify brother or sister.
彼女は私の乳兄弟で、同じ乳母に育てられた。
She is my milk sibling; we were raised by the same wet nurse.
Compound of 乳 (ち, 'milk') and 兄弟 (きょうだい, 'siblings'), literally 'milk siblings'. The term originates from the historical practice of wet-nursing, where children nursed by the same woman were considered to have a special bond.