Euripides,
Orestes 804-806 (tr. David Kovacs):
This proves it: get comrades, not just blood kin!
An outsider whose character fuses with yours
is a better friend to have than countless blood relations!
τοῦτ᾽ ἐκεῖνο, κτᾶσθ᾽ ἑταίρους, μὴ τὸ συγγενὲς μόνον·
ὡς ἀνὴρ ὅστις τρόποισι συντακῇ, θυραῖος ὢν, 805
μυρίων κρείσσων ὁμαίμων ἀνδρὶ κεκτῆσθαι φίλος.
Proverbs 18:24 (KJV):
There is a friend that sticketh closer than a brother.
Cf. Hesiod,
Works and Days 707 (tr. Hugh G. Evelyn-White):
Do not make a friend equal to a brother.
μὴ δὲ κασιγνήτῳ ἶσον ποιεῖσθαι ἑταῖρον.
Related post:
Family Values.