Mary Mehaffey was mentioned in the will of Rebecca (Alexander) Emmitt. The will was written in Chester County, Pennsylvania on 21 Oct 1785 and was proved on 22 Aug 1792. Rebecca did not specifically say that Mary was her sister, but all the other bequeaths were to family and so the circumstantial evidence is good.
The known parents of Rebecca (documented through wills) were John Alexander and Susanna [ -?- ]. John's will was also in Chester County 1764. Susanna died intestate 1767.
Rebecca evidently also had a sister, Martha, who m. William Porterfield. Porterfield genealogists seem to believe that these Alexander "children" of John & Susanna were born in Ireland. I have seen no evidence of this, but also have not tied these Alexanders to the very large family that lived primarily in Cecil County, Maryland.
Please feel free to contact me at if you would like copies of these wills or any additional information that I might have.