αδελφαι G79 N-NPF
αδελφας G79 N-APF
αδελφη G79 N-NSF

what is the difference? the first two may be plural... but i dont know stongs abbreviations (N= noun? NPF...? APF...? NSP....?)

which one best fits the context of two sisters?

thanks

They are all the same word = "female friend"

The difference is in the grammar. Greek has inflected endings for sing/pl, case, etc.

N-NPF = Noun - Nominative case, Plural, Feminine
N-APF = Noun - Accusative case, Plural, Feminine
N-NSF = Noun - Nominative Case, Singular, Feminine

G79 is Strong's Concordance Number (Greek word 79).

As stated above, these are all different forms of the same word.