She was listed in the 1850 census as Emaline, age 12, born Ohio, and in the 1860 census as Emaline, age 18, born Ohio. If the earlier birthdate is correct, she is probably a daughter of Andrew’s first wife, and if the later one is correct, she may be a daughter of his secoind wife, Sarah Brooks.

She may be the Emily Arnold listed in the following family:

1870 census of Greenup Co., Ky, precinct 5, enumerated 28 July, lists on p. 55 as family #406/400:
Rose, James, 61, farming, b. Penn; Josephine, 34, b. WVa; Samuel, 30, b. WVa, iron ore miner; Calvin, 23, b. WVa, works at furnace; Arnold, Emily, 28, b. Ohio, seamstress; Robert, 2, b. Ky.

The birthplace of Ohio fits, as does the age 18 listed in the 1860 census.