Answer
After her father's remarriage, Claudia Octavia's stepsister was Nero. Claudia Octavia was the daughter of the Roman Emperor Claudius and his third wife, Valeria Messalina. Messalina was executed in 48 AD, and Claudius remarried the noblewoman Agrippina the Younger. Agrippina was the mother of Nero, and he and Claudia Octavia became stepsisters. Nero was later adopted by Claudius and became the Roman Emperor.