Complex sentence: Octavia's mother was executed in 48, for conspiring to murder her father, the emperor Claudius, who subsequently married his niece (and Octavia's first cousin) Agrippina the Younger.
Split into simpler sentences: Octavia's mother was executed in 48, for conspiring to murder her father. Claudius subsequently married his niece (and Octavia's first cousin) Agrippina the Younger.