For the first major event, I'm going to go with the opening death and burial of Miss Mary Finch. Without her death, Isabel and Ruth would never have been sold over to the Lockton family. It was always Miss Finch's plan to give Isabel and Ruth their freedom, but Miss Finch's brother sells them instead. That sale propels the plot forward and begins much of the conflicts to come.
I would say that the second main event is Curzon's news to Isabel that his owner might be able to help her get away from the Locktons and earn her freedom. In exchange, Isabel must spy on the Locktons who are Loyalists. In exchange for her freedom, Isabel agrees.
A third major event is kind of a series of events. The first part is Madam Lockton discovering that Isabel is a spy. Isabel's courage is growing and she works to defy the Madame. Madame Lockton locks Isabel up in response. The second part is Isabel's escape from the Lockton house and subsequent rescue of Curzon.