أَبَوَانِ = two fathers (nominative); أَبَوَيْنِ = two fathers (accusative/genitive); آبَاءٌ = fathers.
إِخْوَانٌ and إِخْوَةٌ are both plurals of أَخٌ.
Important limitation: these dual/plural forms follow their own number patterns; do not force the singular أَبُو/أَبَا/أَبِي pattern onto them.
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The Five Nouns: Core Case Forms
أَبَوَانِ = two fathers (nominative); أَبَوَيْنِ = two fathers (accusative/genitive); آبَاءٌ = fathers.
إِخْوَانٌ and إِخْوَةٌ are both plurals of أَخٌ.
Important limitation: these dual/plural forms follow their own number patterns; do not force the singular أَبُو/أَبَا/أَبِي pattern onto them.