Tradition seems to identify Rebecca as the daughter of William McLelland and Mary Ross. Is she referenced in the will of either?
North Carolina marriage records on Ancestry.com indicate that Rebecca McLelland married William McLelland, son of Nicholas, on 10 April 1772. I am not aware of any records of her birth or death, but a reasonable approximation of birth would be c. 1750. The evidence of the grave of Rebecca and William's first child, John, is that he was born ca. 1772. See here: https://www.findagrave.com/memorial/20626523/john-mclelland