Sincerity rhymes with:
celerity, clarity, dexterity, disparity, familiarity, geraghty, hilarity, irregularity, parity, peculiarity, polarity, popularity, posterity, prosperity, rarity, regularity, similarity, solidarity, temerity, unfamiliarity, verity
Sincerity sounds like:
sanskrit, snickered, snookered, sungard