ἀδελφός
Etym. 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.
II. adj., ἀδελφός, ή, όν, "brotherly or sisterly", Trag., Plat.
2. like Lat. geminus, "gemellus", of anything "in pairs", "twin", Xen.:—then, "just like", c. gen. or dat., ἀδελφὰ τῶνδε, ἀδελφὰ τούτοισι Soph.