Based on the research of Karen Campe Mateyka, Colonel Benjamin Van Stephenson family genealogist, Margaret is placed as the daughter of James and Maria/Mary (Reed) Stephenson. She married Thomas Sterrett.

Perhaps given name, Margaret, was in memory of Mrs. Stephenson’s mother, Margaret Reed nee Floyd.

Probable narrative:
Born circa 1767 Margaret Stephenson married Thomas Sterrett circa 1785 in PA (Colony). Settling in Erie, PA, Thomas bought land there, but after problems and litigation regarding this land purchase, the Sterrett family moved west to the Kaskaskia area in Randolph County, IL Territory. Thomas ‘Sterrit’ was Justice of the Peace in Randolph County, IL, by 10 January 1815. A cursory review of census entries suggests that by 1820 Thomas was in Monroe County, IL, and probably a widower at that time. However, the 1825 Randolph County, IL, lists a Thomas Sterrett there.

Illinois Historical Collections, p. 312 viewed at Ancestry, indicate that both a Thomas Sterrett and William Sterret, adjacent entries, were living in Monroe County, IL. Was William a son, brother, or other relative of Thomas? Of interest, listed on the same page, is Benjamin Stephenson of Madison County, IL, believed to be a brother-in-law of Thomas Sterrett:
Title: Illinois Census Returns, 1810 [and] 1818
Chapter: Index to Census of 1818
Page: 312

Sterrett children, believed to be those of Thomas & Margaret, were traced at the IL Online Databases. Because Thomas Sterret did reside in Randolph County, IL, and Monroe County was formed in 1816 out of Randolph and St. Clair counties, the then Illinois Territory, marriage records of Randolph, St Clair, and Monroe Counties were checked for Sterret marriages. The following were probably their children, or perhaps children of a William Sterrett, present on the 1818 IL Census:

Moria Sterrett married Michael Griffean, 1812, Randolph County, IL
William Sterret married Polly Moore, 1816, St. Clair County, IL.
Sally/Sarah Sterrett married Moses Lock, 1819, Monroe County, IL.
Margaret Sterrett married Elisha Pelham, 1819, Monroe County, IL.
Washington Sterrett married Mary Patterson, 1820, Randolph County, IL.
James married Mary Ann Canada, 1821, Monroe County, IL
Thomas married Lucy Dean, 1823, Monroe County, IL.

Perhaps Margaret Sterrett nee Stephenson died sometime before 1810 and Thomas Sterrett died circa 1818-1825. Not conclusively, descendants are in Monroe and Gallatin counties, IL, and CA. All information must be reviewed by descendants.
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THE TERRITORIAL PAPERS OF THE UNITED STATES
COMMISSIONS AND APPOINTMENTS ISSUED BY THE ILLINOIS TERRITORIAL GOVERNOR
COMPILED BY CLARENCE EDWIN CARTER
transcribed by Kim Torp
Illinois State Genealogical Society Quarterly, Vol. XII, No. 1, Spring 1980:
http://genealogytrails.com/ill/territorialpapers.html
1815

1815 Jan. 10 - William ALEXANDER - County Treasurer - Randolph Co.
1815 Jan. 10 - Thomas STERRIT - Justice of Peace - Randolph Co.
1815 Jan. 24 - Matthew DUNCAN - Justice of Peace - Randolph Co.
1815 Feb. 8 - Hezekiah WEST - County Treasurer - Johnson Co.
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1820 United States Federal Census
about Thomas Sterrett
Name: Thomas Sterrett
County: Monroe
State: Illinois
Enumeration Date: August 7, 1820
Free White Males - 10 thru 15: 1
Free White Males - 45 and over: 1
Free White Females - 10 thru 15: 1
Number of Persons - Engaged in Agriculture: 2
Free White Persons - Under 16: 2
Free White Persons - Over 25: 1
Total Free White Persons: 3
Total All Persons - White, Slaves, Colored, Other: 3
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Prairie du Rocher--1825 Census Index of Randolph County, Illinois
PRAIRIE du ROCHER Township
Pierre Lecompte and Patrick Larner, Supervisors of Roads, Henry Barbeau, Assessor, Thomas Sterritt and Archibald McNabb, Overseers of the Poor. The Village of Prairie du Rocher had 38 heads of household. The total number of inhabitants was 202. Included were 28 Slaves and 10 free persons of color.
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http://genealogytrails.com/ill/monroe/earlymarriages.html
Daughter Margaret married in 1819:
Illinois Marriages to 1850
Name: Peggy Sterrett
Spouse: Elisha Pelham
Marriage Date: 3 May 1819
Marriage County: Monroe
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An obituary of the Sterrett grandson:
Benjamin Milton Pelham Born August 23, 1834 in Waterloo, Illinois to Elisha and Margaret Pellham Married: November 15, 1856 CHILDREN: Ida M., Emma, Walter, John, Died: April 8, 1907 in Garden City, Finney, Kansas (after an illness of several months) (Courtesy of PSGOOD and Davidtodd, June 2020, Ancestry Message Board)