An unrelated male considered to share such a strong relationship generally.
An unrelated male considered to share a brother-like relationship established by a ceremonial sharing of blood in some cultures.
A brother by blood: a brother-german or half-brother, as distinguished from step-brothers.
A relative by marriage (or through affinity).
Sometimes specifically a parent-in-law of one's child, for which the dedicated terms co-mother-in-law, co-father-in-law are rare.