Complex sentence: Warwick's sister, and therefore Edward IV's niece, Margaret Pole, Countess of Salisbury was executed by Henry VIII in 1541but by this period the cause was more religious and political rather than dynastic.
Split into simpler sentences: Warwick's sister, and therefore Edward IV's niece, Margaret Pole, Countess of Salisbury, was executed by Henry VIII in 1541. By then, the cause was more religious and political rather than dynastic.