Complex sentence: Lupin told Harry that after James matured and changed his attitude, Lily started dating him in their seventh year, and married soon after leaving Hogwarts with Sirius acting as best man at their wedding.
Split into simpler sentences: Lupin told Harry that after James matured and changed his attitude, Lily started dating him in their seventh year. They married soon after leaving Hogwarts with Sirius acting as best man at their wedding.