I found the marriage of Samuel Abernathy to Mary Ann McAdams in Clermont County, Ohio, 31 October 1833. He was named a widower. Both were residents of Williamsburg Township, Clermont County. Married by justice of the peace Axel Boggs.
So Casandra Purdum's children were John and James Abernathy. The other two children, Sarah and Isaac, on the 1850 census were Mary Ann's children, so were not named as heirs of Benjamin Purdum.