yes she was
No, Mary Shelton was Anne Boleyn's cousin. There is a link below to an article on Margaret and Mary Shelton.
They were sisters.
Anne had one sister (Mary Boleyn) and one brother (George Boleyn)
Anne and Mary Boleyn were sisters. Both were mistresses to King Henry VIII, but Anne became queen. Mary died in her early forties in 1543; Anne Boleyn was executed in 1536.
Queen Mary I of England was the daughter of Henry VIII and Catherine of Aragon