Here we have Margaret Ker, wife of Walter Ker, as being the daughter of Rev. William Johnstone and Helen Cullen, but there is no source material to support this.
In A Quest for the Ancestry of Walter Ker, p. 36, by Randolph and Dora V. Kerr; https://www.familysearch.org/library/books/viewer/188477/?offset=0#... [paywall], they state that "we do not know the name of Walter Ker's wife, other than it was Margaret.
In Genealogical and Personal History of Western Pennsylvania, p. 890, by John Jordan; https://archive.org/details/genealogicalpers02injord/page/890/mode/2up, he does not attribute a last name to Margaret.
In The Kerr Clan of New Jersy, p. 4, by William Armstrong, he states that Walter's wife was named Margaret, and that she may have been the daughter of James Johnstone. However, on p. 5, Armstrong discusses 5 sons of the Kers, including James, of whom "there is no record except this allusion: 'In 1691 James Johnstone deeded land to Walter Ker in right of his second son, James Ker.'"
There is a Genealogy.com forum, from 2007, (https://www.genealogy.com/forum/surnames/topics/ker/89/) with an article by Timothy Taylor, as a response to Douglas Somerset, in which Taylor states that Walter's wife was Margaret Johnstone, but he provides no proof. In 2012 Douglas Somerset had an article in Scottish Reformation Society Historical Journal, entitled Walter Ker and the "Sweet Singers" in which he avoids any mention of a wife for Walter.
Leah Parker, On the Trail of the Johnstone, Ker, and Barclay Families; https://www.clanbarclayinternational.org/uploads/1/3/0/9/130953147/..., has a good synopsis of all the works above mentioned, as well as others, including a transcript of the land deal by James Johnstone for the benefit of a young James Ker. She also opines that it is unlikely that Margaret was a daughter of James Johnstone given their respective ages, and that Margaret Ker was not the known sister of James and John Johnstone, as that woman remained in Scotland. Margaret, wife of Walter Ker, is most likely connected to the Johnstones, but there is not enough evidence to show in what way.
Absent any conclusive evidence, I think Margaret Ker should be shown as "parents unknown."
David Prins, what say you?
Erica Howton is this too far south for your expertise?
