Nancy Epps Chisum was actually the Aunt of John Simpson "Cow John" Chisum, not his sister.
James Chisum m. Betsy Gibbons, daughter of Thomas Gibbons and Ann Epps. Their children are as follows:
Clairborne m. Lucinda Armstrong
John G m. Eliza Pirtle
Clairborne Chisum m. Lucinda Armstrong. Their children are as follows:
John Simpson "Cow John"
None of my family papers give me any additional information about Nancy Epps Chisum aside from her place in the family tree, so thank you for the jumping off point. =D