It is unlikely that Elizabeth was the sister of Shadrach Simcock/Simcox of Venango County. Shadrach's father, William Simcock, was in Kent County, Maryland until about 1775 when he sold all of his property and left for parts unknown. I suspect he might have been a loyalist and headed for Canada, but I haven't proved it. In any case, Shadrach and his brother, William make a sudden SHORT appearance in Chester County about 1790 for 10-15 years before moving to western PA. Also Shadrach is believed to have a sister named Elizabeth Simcock but she is the one who married Azariah Bostwick and lived in Baltimore County, Maryland.
I don't have absolute proof, but your Elizabeth Simcox/Simcock was probably the daughter of Samuel Simcock and Sarah Trego. If so, she would have been born in West Bradford Township, Chester County, PA where Samuel lived in 1757. In 1757, there were only two Simcock men in eastern PA of the right age to be her father, the aforesaid Samuel and his brother, Benjamin. Benjamin's daughter, Elizabeth, was born 3 Jan 1751 according to church records, so that only leaves Samuel. This conclusion is also supported by the fact that John Parsons and Elizabeth's 3rd & 4th children were named, Samuel and Sarah.
Samuel Simcock was the son of Benjamin Simcock and Hannah Waln. Sarah Trego was the daughter of James and Elizabeth Trego.
Another possibility for the parentage of Elizabeth is that she is the illegitimate daughter of Alice Simcock:
From Bradford Monthly Meeting, Chester County, PA: 11-17-1757 Aaron Mendenhall, Jr., guilty of unseemly behavior (adultery) with Alice Simcock [Aaron was married to Mary Woodward, sister of Abraham Woodward]
Minutes of Goshen Monthly Meeting 3-20-1758: A committee was appointed to prepare testimony against Alice Simcock for fornication.
12-12-1760: Bradford Preparative Meeting informed that "Alice Simcock accuseth Abraham Woodward of being father of her bastard child and complains of him for accomplishing his marriage with another woman by the assistance of a priest."
Abraham and his wife soon left for NC. Abraham temporarily returned to Bradford Meeting in 1765 to confess and was reinstated as a Quaker.
Elizabeth was born at about the right time to be this child of Alice. Based on time frame and location, Alice is almost certainly the daughter of Samuel SIMCOCK and Sarah TREGO. If Elizabeth is this child, she was likely reared by her grandparents, Samuel and Sarah, and would explain the names of Elizabeth's 3rd & 4th children.
Samuel first appears as a single freeman in the 1735 and 1736 tax lists of Whiteland Township, Chester County, PA. He is in the tax list of Westtown Township in 1737 and 1738 and in Chester Township tax lists for 1739 and 1740. The Commissioners Book shows that he operated a public house (tavern) in Chester in 1740-1741. He was again in Whiteland Township in 1747-1751. He is in West Bradford Township in 1756, 1757, 1758, 1762 and 1763, in East Bradford in 1765 to 1768, and in Westtown township in 1775.
There is no known relationship between the Simcock family of Kent County, Maryland and the other Simcock family that had been established in Chester County since pre-1700. The fact that Shadrach lived in Chester County for a few years before moving to Venango County is probably just coincidence. He just happened to be of marrying age when he was in Chester County, so that is where he married.