sister has 1 distinct synonym spread across 1 of its 3 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 brother.
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 female person who has the same parents as another person
“my sister married a musician”
Synonyms, antonyms and the broader and narrower relations for sister come from Princeton University’s WordNet. Full sources and licences.
(Roman Catholic Church) a title given to a nun (and used as a form of address)
“the Sisters taught her to love God”