Hmm interesting could he push for the girl to be recognised as Queen then? Alternatively, could as a way to broker peace, Alfonso de la Cerda marry Isabella?
What is in for him ? With a « Alfonso X » settlement, he gets an independent realm plus the regency. That is not a bad situation for higher ambition. Isabella and Alfonso are first cousins. A papal dispensation is needed but, since her own parents dispensation never came, Isabella is a bastard in the eyes of the Church. Not a chance in Heaven the Pope would sign for the match.