"After all, she looked rather like Mark Smeaton, the musician executed for adultery with Anne Boleyn"

Mark Smeaton, a musician in the court of Henry VIII during his marriage to Anne Boleyn, was executed, along with four other men for treason, that is, adultery with the Queen, Anne Boleyn.  It is generally accepted that he, and the others, including Anne's own brother, George Boleyn, were used by Thomas Cromwell in order to bring Anne down and thereby free Henry VIII to marry Jane Seymour. 

 

Mary's attitude towards her younger half-sister Elizabeth hardened once she became Queen, convincing herself that Elizabeth was not the daughter of Henry but of Mark Smeaton, this in spite of the striking resemblance between Elizabeth and Henry.