Brother vs monk

Brother and monk both are nouns.

Word	Noun	Adjective	Verb	Adverb
Brother	Yes	No	No	No
monk	Yes	No	No	No

As nouns, monk is a hypernym of Brother; that is, monk is a word with a broader meaning than Brother:

Brother: (Roman Catholic Church) a title given to a monk and used as form of address
monk: a male religious living in a cloister and devoting himself to contemplation and prayer and work

Other hypernyms of Brother include monastic.

Brother (noun)	monk (noun)
(Roman Catholic Church) a title given to a monk and used as form of address	a male religious living in a cloister and devoting himself to contemplation and prayer and work