يَا أَيُّهَا النَّبِيُّ contains النَّبِيُّ, which is definite with ال.
يَا نِسَاءَ النَّبِيِّ contains the idafah نِسَاءَ النَّبِيِّ, definite through its definite mudaf ilayh.
Calling does not replace the original source of definiteness.
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Definite with Al or Idafah
يَا أَيُّهَا النَّبِيُّ contains النَّبِيُّ, which is definite with ال.
يَا نِسَاءَ النَّبِيِّ contains the idafah نِسَاءَ النَّبِيِّ, definite through its definite mudaf ilayh.
Calling does not replace the original source of definiteness.