sternness vs unpermissiveness

sternness and unpermissiveness both are nouns.

Word	Noun	Adjective	Verb	Adverb
sternness	Yes	No	No	No
unpermissiveness	Yes	No	No	No

As nouns, unpermissiveness is a hypernym of sternness; that is, unpermissiveness is a word with a broader meaning than sternness:

sternness: uncompromising resolution
unpermissiveness: a lack of permissiveness or indulgence and a tendency to confine behavior within certain specified limits

Other hypernyms of sternness include restrictiveness.

sternness (noun)	unpermissiveness (noun)
uncompromising resolution	a lack of permissiveness or indulgence and a tendency to confine behavior within certain specified limits
the quality (as of scenery) being grim and gloomy and forbidding