Why wasn't Mary charged/executed?

*Spoilers*

I like the movie and I love the book, but something that has always confused me is why Mary did not get charged and executed in the same manner as Anne and George. In the book, she flat out says (I forget to who, either Anne and George or William) that she is just as guilty as they were, especially since the three of them spent so many nights together in Anne's chambers. Additionally, in the book, Mary and George get the potion to abort one of Anne's babies, and William and George bury it. Outside of the incest and affairs, it seems that Mary should have been on trial as well.

The only excuse I can think of is that Mary was always in King Henry VIII's good graces. He always considered her very simple and sweet. Perhaps since she never did any directly to hurt him, he did not consider her to be guilty.