She was the daughter of Thomas ROPER and wife Mary.
Thomas ROPER Anne Arundel County19 Sep. 167726 Oct. 1677 To wife Mary, execx. 1/2 plantation during life. To dau. Mary and hrs. 300 A. "Ropers Increase" in Balto. Co., and 1/2 plantation at majority.Wife or dau. afsd. dying without issue, deceased's portion to revert to survivor. Overseers: Dr. Wm. JONES, Wm. MITCHELL. Test: Wm BOWMAN, Thos. JEFFE, Hugh PRITINGALE.5:313 [Baldwin, *Maryland Calendar of Wills*, 1:197.
The widow Mary secondly married to John HAMMOND (HAMMON) between 26 Oct. 1677 when Thomas ROPER's will was written and 1 Nov. 1678.On the latter date it was stated that Thomas ROPER of Anne Arundel County in his will requested if his wife should marry, to secure his child's estate.Since that time his widow married John HAMMON of said county, who was likely to embezzle the estate and therefore Mary HAMMON, the relict of Thomas ROPER was to render an account. [MD Testamentary Proceedings 10:315, FHL microfilm 0,012,933]
As you note, Mary (ROPER) HOWARD died on 24 Dec. 1714.Her husband Cornelius HOWARD died 23 Feb. 1716/7. [St. Anne's Parish Register]In his will written 18 Sep. 1715 and proved 19 March 1716/7, Cornelius HOWARD (among other bequests) left his son Thomas HOWARD 250 acres of "Ropers Increase." [Maryland Wills 14:275, FHL microfilm 0,012,846] His being in possession of the tract "Ropers Increase" is what ties his wife Mary to being the Mary ROPER who was willed the tract by her father.
Mary (ROPER) HOWARD did not have a will.The will you mentioned as being proved on 6 April 1721 was the will of her mother Mary (-?-) ROPER HAMMOND. [MD Wills 16:366, FHL microfilm 0,012,846]Her bequests were to her HOWARD grandchildren [not stated, but the children of Cornelius HOWARD and her daughter Mary ROPER].
Mary (ROPER) HOWARD had no blood siblings, only step-siblings, children of her step-father John HAMMOND by his first wife (given and surname unknown).