In History: While there were much more prominent women named Mary in the actual Salem witch trials, Mary Sibley is mentioned once in the context of Tituba. According to Tituba, Reluctant Witch of Salem, Mary Sibley invented the recipe for the "witch cake" and helped Tituba bake it. It's interesting that the two crossed paths in history, especially since they're so entwined in the TV series.
On Salem: After her lover, John Alden, leaves for war, Mary Sibley (Janet Montgomery) sacrifices her unborn child and becomes a witch. She marries George Sibley to attain a position of power and manipulate the witch hunt. Mary still loves John Alden and has just found out her child is still alive.