Translation:His sister is an energetic young woman.
kaikuahine in general, refers to "younger sister of an older brother." So kona has been translated correctly as "His" instead of "Her."
When referring to a younger sibling of the same sex, it would be kaikaina.
If she is the sister-in-law to a male, then the reference should include whether she is older or younger than the brother-in-law. She would either be wahine makua or wahine 'opio.