The Feminine Genitive

Feminine nouns form the genitive singular by simply adding -ς to the nominative: μητέρα → μητέρας, πόρτα → πόρτας, αδελφή → αδελφής, ώρα → ώρας. Nothing else changes and the stress does not move. The genitive plural is where the work is: the ending is -ων, and for the ordinary -α / -η feminines it pulls the tonos all the way to the ending - πόρτα → πορτών, θάλασσα → θαλασσών, ώρα → ωρών. A feminine that is already end-stressed has nothing to move: αδελφή → αδελφών keeps the tonos exactly where the nominative had it. The kinship nouns are the exception, and they are among the words you use most: μητέρα → μητέρων keeps the tonos on the penult and moves nothing, and its masculine partner πατέρας → πατέρων behaves identically. Both descend from the ancient third declension rather than the -α class they now resemble, so «μητερών» is simply wrong.