Why did Claudius change Roman law before marrying Agrippina the Younger?

Claudius changed Roman law specifically to permit his marriage to Agrippina the Younger, who was his niece. Under existing Roman statutes, such a union was considered unlawful; therefore, he had the law altered to facilitate the marriage in 49 CE.1

This marriage was a significant political move, as it allowed Agrippina to exert considerable influence over the emperor. To satisfy her ambitions, Claudius was induced to adopt her son, Lucius Domitius Ahenobarbus—who later became the emperor Nero—as his heir, a decision that came at the expense of his own son, Britannicus.12