stepsister vs half-sister

stepsister and half-sister both are nouns.

Word	Noun	Adjective	Verb	Adverb
stepsister	Yes	No	No	No
half-sister	Yes	No	No	No

As nouns, stepsister and half-sister are synonyms defined as:

stepsister and half-sister: a sister who has only one parent in common with you

Other synonyms of stepsister include half sister.

stepsister (noun)	half-sister (noun)
a sister who has only one parent in common with you	a sister who has only one parent in common with you