Complex sentence: Dr. Derek Shepherd (Patrick Dempsey) was the chief of neurosurgery, later replaced by his sister, Dr. Amelia Shepherd (Caterina Scorsone).
Split into simpler sentences: Dr. Derek Shepherd (Patrick Dempsey) was the chief of neurosurgery. He was later replaced by his sister, Dr. Amelia Shepherd (Caterina Scorsone).