Feminine issue one's sister has (6)

I believe the answer is:

nieces

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'has' is the definition.
Although both the answer and definition are plural nouns, I don't understand how they can define each other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for nieces that I've seen before include "They're related" , "Young female relatives" , "Relations" , "They're family" , "Relative girls" .)

I've seen this clue in The Irish Times.

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