Rebecca was a descendant of the Calvert family of Prince George's County, MD. Her grandmother, Rosalie Stuart [b. 1801] was the daughter of Eleanor Calvert Stuart (1784-1823) and Dr. David Stuart of Fairfax County, VA. Eleanor Calvert Stuart was the widow of John Parke Custis, George Washington's stepson. Rosalie Stuart had four sisters, Eleanor Custis, Ann, Sarah, and Ariana, two brothers, William and Charles, and was the step-sister of Elizabeth, Martha, Eleanor, and George Washington Parke Custis, the grandchildren of George and Martha Washington. When Rosalie's sister Eleanor Custis Stuart died without children, she left a portrait of their sister Ann, by Gilbert Stuart, to her great-niece Rebecca Webster. This portrait is now in the NGA [1942.8.22]. [Compiled from sources and references recorded on CMS]
Johnson, Robert Winder. The Ancestry of Rosalie Morris Johnson. Philadelphia, 1905:30, 41.
Torbert, Alice Coyle. Eleanor Calvert and her Circle. New York, 1950: opposite p. 1 (genealogical table)