In real life, Isabelle de Broyes was married three times before the age of 21, when she married Thurstan, had one child and then lived and died in Normandy. In the story she suffers from extreme PTSD from her previous marriages; it’s very possible that such abuse takes place as this was very common in those times.  Because she is so wealthy, and a descendant of Charlemagne, she has the resources to build an Abbey and run away to live there. In Piercing Time she’s spiteful and a little cruel to Cynewyn, but that is not the final act in her story.