Is is true that Mary Boleyn never saw or went to visit her sister and brother while they were held in the tower? And is it also true that Anne’s niece Catherine witnessed her execution?
That is true, Mary Boleyn did not visit her siblings in the Tower and stayed away - a sensible choice.
It is now thought that Catherine Carey did not serve her aunt, Anne Boleyn, in the Tower and did not witness her execution. There is certainly no evidence to back that up. In 1540, at the age of 16, she became a maid-of-honour to Anne of Cleves and in that same year, on the 26th April 1540, Catherine married Francis Knollys, a favourite at Henry VIII’s court and one of the men sent in 1539 to attend Anne of Cleves on her arrival in England.
After the annulment of the Anne of Cleves marriage, Catherine went on to become maid-of-honour to Catherine Howard. Her husband became an MP in 1542, was knighted in 1547 during Edward VI’s reign and became a good friend of the Protestant William Cecil. The Knollys’ Protestant beliefs led to them leaving England in 1553 when the Catholic Mary I came to the throne and living in exile in Germany. In 1558, Catherine and Francis returned to England and Francis became a member of Elizabeth I’s privy council and then Vice-Chamberlain of the Household and Captain of the Halberdiers. Catherine became one of Elizabeth’s ladies of the privy chamber, serving her cousin (or half-sister!) as the Chief Lady of the Bedchamber.