Elizabeth Draper Wager, sister of David Draper.

The only sibling of David Draper's that I am absolutely certain of is his sister Elizabeth Draper. In the will of their father, John Draper, only two children are named- David Draper and Elizabeth, wife of Thomas Wager. This does not mean, of course, that other children were not born to John and his wife- just that most likely only David and Elizabeth were alive in 1811 at the time that the will was written.
From the burial register of St. Andrews, Enfield, we know that Elizabeth Wager was 32 years old when she died in 1817. This places her year of birth as c. 1785.
When looking for baptisms of Elizabeth Drapers around this time with father John Draper, we come across a perfect fit....

Elizabeth Draper, baptised April 23, 1785, St. Andrews, Enfield, daughter of John and Mary Draper.

The only other child baptised at St. Andrews, Enfield, to John and Mary Draper was another daughter, Anne, who was baptised on May 17, 1783. I searched through every page of the St. Andrews baptismal register for a twenty year period, and could not locate any further children born to this couple, most particularly no David Draper was found.
An Anne Draper was buried at St. Andrews, Enfield, on April 21, 1787, but no age was given, so we can't know if she was the child of John and Mary.

As far as we know, David Draper was born to John and Mary Draper in c. 1787-88, at Enfield. We also know that his father John had been widowed by November 1795, as this is when he remarried widow Frances Banks.
There was only one Mary Draper buried at St. Andrews, Enfield, in the period 1787 to 1795...she was buried on 31 Aug 1791, no age given. We know that John Draper's wife was buried at St. Andrews, Enfield, because in his will he requests to "be interred in the church yard of this parish as near to my former wife as possible."
Elizabeth and David would have been aged only six and four or five respectively if this Mary Draper was their mother, and were aged about ten and eight when their father remarried.

Elizabeth Draper married wheelwright Thomas Wager at the church at Monken Hadley on August 12, 1804, aged 19 years. Thomas had been born c. 1782, so he was about 22 years old when he married Elizabeth.

The couple lived at Chase Side, Enfield, and their five children were all born in Enfield and baptised at the Draper family church, St. Andrews. Three daughters and two sons were born between 1806 and 1817:-

MARY ANN WAGER: born June 21, 1806.
CHARLOTTE WAGER: baptised November 16, 1808.
JOHN WAGER: baptised August 16, 1811.
THOMAS WAGER: baptised June 13, 1813.
ANNE WAGER: baptised April 14, 1816.

A year after the birth of daughter Anne, Elizabeth Draper Wager died, aged only 32 years. She was buried at St. Andrews, Enfield, on July 25, 1817.

Thomas Wager was left to raise five children aged between 11 and 1, yet I can find no record of him remarrying after his first wife's death, which is what the majority of men in his situation would have done.

Thomas Wager, widower, died in 1829, aged 47 years. He was buried at St. Andrews, Enfield, on December 27, 1829.