Complex sentence: Her grandfather was killed fighting against her uncle, Edward IV of England at the Battle of Barnet; her father had then been created Earl of Salisbury and of Warwick; he was already Duke of Clarence.
Split into simpler sentences: Her grandfather was killed fighting against her uncle, Edward IV of England, at the Battle of Barnet. At that time, her father was then created Earl of Salisbury and of Warwick; he was already Duke of Clarence.