How does Sylvie Mongeau assist John Fleming in the novel, A World of Curiosities?

Sylvie Mongeau claims to be John Fleming’s wife even though no marriage certificate is located. Sylvie helps Fleming put together his plan to kill Gamache while he is in jail by pretending to be a woman named Lillian Virginia Mountweazel. As Mountweazel, she stays in Three Pines and steals the elephant from the bed and breakfast. She arranges for the letter to be mailed to Billy’s ancestral home and forwarded to Billy’s home with Myrna.

In the present, the people of Three Pines believe Sylvie is the wife of the minister. Sylvie is suffering with cancer. She dies during the time in which the novel is set. Gamache is correct when he suspects that Fleming, disguised as the minister, murdered his wife.