WordNet records no synonym and no antonym for tongue twister. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
an expression that is difficult to articulate clearly
“`rubber baby buggy bumper' is a tongue twister”
Synonyms, antonyms and the broader and narrower relations for tongue twister come from Princeton University’s WordNet. Full sources and licences.