Your Cynthia Ann McPeters is almost surely the daughter, listed as Syntha, age 9, in the 1880 Census of Kimble County, TX. She was the daughter of John and Lorraine McPeters of Kimble County who had older sons, Wylie and John L. McPeters. This family had previously lived in the area of Gonzales County, TX, but, the father, John McPeters, age 51 in 1880, was born in Arkansas, a son of a Thomas L. McPeters.
There are questions about exactly which McPeters Family that Thomas L. McPeters came from, but he was born in Tennessee, and apparently his father was also a John McPeters who had previously married a Rachel Robinson in Grainger County, TN, in 1803. The family later lived in middle Tennessee where Thomas was born in Williamson County.
There are stll questions regarding exactly which of the McPeters families of that Eastern Tennessee area was the father of that John. It was most likely a Joseph McPeters who had been born in Western North Carolina and later moved to Eastern Tennessee and then to Giles County in Middle Tennessee. That Joseph McPeters also appeared in the 1830 Census of Arkansas living alone but near the home of Thomas L. McPeters.
However, there is also a possibility that John McPeters who married Rachel Robinson in Grainger County, Tennessee, in 1803 was a son of Joseph's brother, David McPeters, who also lived in that area of East Tennessee (Morgan County) for a few years before 1820.
Both David and Joseph McPeters were apparently sons of a Charles and Mary (McDowell) McPeters of old Burke County, North Carolina. Charles and Mary had come from Northern Ireland to the Irish Settlement along the Pennsylvania/Maryland border and then moved down the Shenandoah Valley to Augusta County, Virginia, and later on to old Burke County, North Carolina, along with his McDowell in-laws.
It now appears that Charles McPeters was one of several youg McPheeters families who came to America in the 1720s and first settled in that Irish Settlement, but later moved South to Virginia and North Carolina. In Virginia and several times before the Revolutionary War Charles signed his name as "McFeeters" or "McPheters", but then the entire family chose the spelling "McPeters" about the time of the Revolutionary War.
I have established a McPheeters Surname DNA Project that indicates that while descendants of son, David McPeters, match our McPheeters DNA pattern excatly and thus almost surely descend from the same common ancestor, the DNA pattern for descendants of Joseph do not match the McPheeters/McPeters pattern, thus raising the question of whether Joseph was actally a blood son of that Charles McPeters family or perhaps an infant orphan from another family that was taken by the McPeters family and raised as their own son.
Much of this information comes from Census records and other bits of information, but not from firm family sources or documents such as birth and marriage records. Thus this information is not "Proof" of the relationships, but the circumstantial evidence is quite strong.