ἀδελφός α copul. , δελφύς; cf. Lat. couterinus
ἀδελφοί are properly sons of the same mother:
I.as Subst., ἀδελφός, ὁ, voc. ἄδελφε (not -φέ), ionic ἀδελφεός, epic -ειός:—a brother, or generally, a near kinsman, ἀδελφοί brother and sister, like Lat. fratres, Eur.; ἀδελφεοὶ ἀπ᾽ ἀμφοτέρων brothers by both parents, i. e. not half-brothers, Hdt.
2.a brother (as a fellow Christian), NTest.