Complex sentence: Voltaire claimed that the prisoner was a son of Anne of Austria and Cardinal Mazarin, and therefore an illegitimate half-brother of King Louis XIV, the sincerity of this claim is uncertain.
Split into simpler sentences: Voltaire claimed that the prisoner was a son of Anne of Austria and Cardinal Mazarin, and therefore an illegitimate half-brother of King Louis XIV. However, the sincerity of this claim is uncertain.