blood brother vs male sibling

blood brother and male sibling both are nouns.

Word	Noun	Adjective	Verb	Adverb
blood brother	Yes	No	No	No
male sibling	Yes	No	No	No

As nouns, male sibling is a hypernym of blood brother; that is, male sibling is a word with a broader meaning than blood brother:

blood brother (noun)	male sibling (noun)
a male with the same parents as someone else	a sibling who is male
a male sworn (usually by a ceremony involving the mingling of blood) to treat another as his brother