Do you mean Don Carlos' Isabella or Clara Eugenia? I assume the former. I was honestly under the impression that male preference primogeniture skipped over daughters of any sons who happened to die before their fathers (assuming they have primogeniture in all the Iberian crowns). Though yes, the problem can be solved by marrying her off (or her dying like her siblings). Not entirely sure how it works.I am pretty sure Isabella would be heiress to Castile, though Philip could solve the problem by marrying her.
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