乳ち兄きょう弟だい

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.

• 彼かの女じょは私わたしの乳ち兄きょう弟だいで、同おなじ乳母うばに育そだてられた