Complex sentence: Edmund was always a loyal supporter of his elder half-brother, King Edward II of England, which placed him in conflict with the queen, Isabella of France (who was Edmund's cousin), and her lover Roger Mortimer, 1st Earl of March.
Split into simpler sentences: Edmund was always a loyal supporter of his elder half-brother, King Edward II of England. This placed him in conflict with the queen, Isabella of France (who was Edmund's cousin), and her lover Roger Mortimer, 1st Earl of March.