Who is Clotilde Arsenault in the novel, A World of Curiosities?

Clotilde Arsenault was the mother of Sam and Fiona. She was addicted to drugs and sold her children as prostitutes. Her children killed her when Sam was age 10 and Fiona was age 13. Sam claimed that it was Fiona’s idea to kill their mother and made him cooperate with her plan. Gamache always suspected that while Fiona might have been the one to hit her mother with the brick, that the plan to kill her was Sam’s idea.