A Killing Season by Priscilla Royal

Baron Herbert's return from crusade should have been a joyous occasion. Instead, he grows increasingly morose, withdraws from his family, and refuses to share his wife's bed. When his sons begin to die in strange accidents, some ask whether Herbert harbors a dark sin for which God has cursed him.
Then the baron suddenly sends for Sir Hugh of Wynethorpe, begging his friend to bring spiritual and secular healers but giving little explanation for the request. Worried about Herbert's descent into melancholy and the tragic deaths, Sir Hugh persuades his sister, Prioress Eleanor of Tyndal Priory, as well as a respected physician, Master Gamel, to accompany him. Although he is pleased when the prioress brings her healer, Sister Anne, he is surprised to find the mysterious Brother Thomas included.
Is there a malign presence at this storm-blasted castle, oddly named Doux et Dur? Tensions spark among family members and soon ignite too among those who came to help. Death's scythe harvests more victims, and it is not long before Ecclesiastes' grim words seem all too apt: there is a season for everything under heaven, including a time to kill....