sincerity vs truthfulness

sincerity and truthfulness both are nouns.

Word	Noun	Adjective	Verb	Adverb
sincerity	Yes	No	No	No
truthfulness	Yes	No	No	No

As nouns, truthfulness is a hypernym of sincerity; that is, truthfulness is a word with a broader meaning than sincerity:

sincerity: the quality of being open and truthful; not deceitful or hypocritical
truthfulness: the quality of being truthful

sincerity (noun)	truthfulness (noun)
the quality of being open and truthful; not deceitful or hypocritical	the quality of being truthful
the trait of being serious
a quality of naturalness and simplicity
an earnest and sincere feeling