From Longman Dictionary of Contemporary Englishfoster brother/sisterfoster brother/sistersomeone who has different parents from you, but who is being brought up in the same family → foster
Examples from the Corpus
foster brother/sister• The two play New York subway cops who also are foster brothers.• I shan't be able to treat you now as if you were my foster sister, Jenny.• Her foster brother's misinformation must be dispelled, but what did she say?• She knew her foster brother was unreliable. 