Complex sentence: Peter had succeeded his father John of Luxembourg, Lord of Beauvoir and mother Marguerite of Enghien, who had been Count and Countess of Brienne from 1394 to her death in 1397.
Split into simpler sentences: Peter had succeeded his father John of Luxembourg, Lord of Beauvoir, and mother Marguerite of Enghien. They had co-reigned as Count and Countess of Brienne from 1394 to her death in 1397.