Philip Mertz definitely had a daughter named Sara, she was named last among his daughters in his will.

From what I know of the birth years for his other daughters, I believe Philip listed his daughters in birth order. Philip’s last four daughters, the presumed four youngest, in that list were (from presumed oldest to youngest):

Margaret who was baptized at Zion Morr’s and that baptism record said only the she was born in 1785.
Gertrude who also was baptized at Zion Morr's Church in 1787.
Anna Mary baptized in 1789.

So I have always assumed that Sara was born maybe 1790 or so. But I never knew what happened to her and the various family genealogists of long ago who identified Philip as my ancestor didn’t have any further mention of her either.

Then in 2019, a woman named Glenda Schaeffer, emailed me and she had done a DNA test that identified Sarah, Glenda’s fourth-great-grandmother, as a possible sister of Jacob Martz (1781-1859) and Mary Magdalena Mertz (1778-1840). Jacob and Mary Magdalena were both the children of Nicholas Mertz (1748-1801). So the first guess would be that Sarah too was a child of Nicholas.

Nicholas in fact did have a daughter named Sarah, likely his youngest daughter. She was named Sally in most of the Orphans Court records following his death including the 1831 document when Peter Eisenhower and wife Sally state they were paid their fair share of Nicholas' estate -- $157.33. She was named Salome in the newspaper notice of her marriage dated 16 Apr 1814. “Married, 27 Mar, Peter Eisenhauer and Salome Marz, both of Middleburg, Union County”. But she finally was named Sarah in the 1850 Census as Peter’s wife; she was 53 years of age. (I suspect from the ages of Nicholas’ other children, she was a few years older than that.)

From Glenda’s research, I learned that her 4G-GM Sarah had married George Miller, that Sarah and George were on find-a-grave with dates 1 Jun 1791-31 Mar 1862 for Sarah. When I entered George Miller and Sarah Mertz, as parents, into Ancestry’s database of Pennsylvania Death Certificates, I got two hits: Elizabeth Smith born 1836 who died in Kelly Township, Union County and Edward Miller who was born in 1827 in Union County and also died in Kelly Township. George and Sarah were buried in Mazeppa which is basically just across Spruce Run from Kelly Township.

Indeed, in 1850, George Miller age 66 and Sarah Miller age 59 lived in Buffalo Township, Union County with quite a number of children including Edward age 23 and Elizabeth 14.

Clearly, I can now accept as fact that a woman named Sarah Mertz born in 1791 married George Miller and they lived most of their adult life in Union County. From Glenda’s DNA, I also know that Sarah was of the Longswamp Mertz family which greatly narrows down the list of possible candidates to be Sarah’s father.

To be totally exhaustive, of course, I would have to consider the other Longswamp Mertzes who came to Northumberland County in the 1780’s. But David, Jacob and Peter settled east of the Susquehanna River. I know the names of all of David’s children and there was no Sarah. I don’t necessarily know the names of all the children of Jacob or Peter but I’m not aware that either had a daughter Sarah. And there is no indication that anyone belonging to any of those Martz lines moved to Union County.

So we’re back to Nicholas and Philip who settled west of the Susquehanna. And again to be totally exhaustive, I suppose we should at least consider momentarily if Sarah was a granddaughter of either of them.

Actually when I first started trying to learn more about Sarah and noticed she lived in Buffalo Township, Union County in 1850, my first thought is maybe Jacob — said by Ancestry to possibly be her brother — maybe was in fact her father. He was the one member of either Nicholas’ or Philip’s family to move to that area. But he was too young and besides he had a daughter Sarah who I think was born about 1835 and married Michael Fetter.

Turning back to Philip, his oldest son was Peter. But Peter was too young you would think to have had a daughter Sarah when he was 17 and his wife 15. He did have a daughter Sarah but I believe she was the Sarah who married Benjamin Straub and from her age in Census was born about 1807.

Sara, daughter of Philip, is the most obvious candidate and I think everything points to the fact that Sarah (Mertz) Miller was Philip’s daughter. Ancestry’s DNA analysis was close to being right. Sarah was not the sibling to Mary Magdalena and Jacob, rather she was their first cousin.