Complex sentence: In 138, when Marcus Aurelius was adopted by Roman Emperor Antoninus Pius, Aurelius ended his engagement to Ceinonia Fabia and became engaged to Antoninus Pius daughter Faustina the Younger, whom he later married.
Split into simpler sentences: In 138, when Marcus Aurelius was adopted by Roman Emperor Antoninus Pius, Aurelius ended his engagement to Fabia. Aurelius became engaged to Antoninus Pius daughter Faustina the Younger, whom he later married.