While this doesn't tell who Mary Lewallen was and whether she was the sister of Sarah Isham or not, this does connect the Ishams and the Lewallens. Wiley and Mary Lewallen are my ancestors. One of their sons was James Calvin (went by his middle name) Lewallen and my great grandfather was his son William Henry Lewallen. Here's a portion of what I have on Calvin:
James Calvin Lewallen was born in North Carolina 1822 or 1823 and is buried in the Lewallen Cemetery in Troy, Texas. Per the 1850 census of Cherokee County, Alabama, he was a mechanic but the 1860 census lists his occupation as a wagon maker. On September 28, 1843, he married Eliza D. Hazel in Benton County (now Calhoun County), Alabama. Eliza was born in Alabama in 1827. Sometime between the taking of the 1850 and the taking of the 1860 census, Eliza died and Calvin married Arametta Isham, born 1834 Tennessee, daughter of William and Sarah Isham.
Do you know where in North Carolina the Isham family is from. It does appear these families traveled to the same areas in Tennessee, Alabama and Texas so it could be they came from the same place in North Carolina. I sure need some help with my Lewallen ancestors so anything you can provide will be appreciated.