brother has 7 distinct synonyms spread across 3 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is sister.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a male with the same parents as someone else
“my brother still lives with our parents”
Synonyms, antonyms and the broader and narrower relations for brother come from Princeton University’s WordNet. Full sources and licences.
(Roman Catholic Church) a title given to a monk and used as form of address
“a Benedictine Brother”