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:

Mary
Clairborne m. Lucinda Armstrong
Lucinda
Rebecca
Nancy Epps
Thomas G
Lavinia
James S
Elizabeth
John G m. Eliza Pirtle

Clairborne Chisum m. Lucinda Armstrong. Their children are as follows:

Mary
John Simpson "Cow John"
James
Thomas J
Pittser
William C
Robert C

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