You may have already performed this exercise, but, if not, it may help you to do so. To the best of my knowledge, this family has no connection to me.
In 1870 and 1880 William and Eliza J. and family were in Osage County, MO (Crawford township, I believe), and the census showed William and Eliza both born in MO with William's parents both born in KY and Eliza's father in KY and mother in VA (which you probably already knew). In 1880, William's family included his brother Charles, about 9 or 10, years younger than he, and, just below their family was Mary G. Alexander, 69, born in KY. Mary's family also includes son John, 41, born in MO, and Mary, 15?, who was listed as daughter but must be John's daughter (1870 census confirms this, and Charles, 30, was also in the family.) The elder Mary and John were both widowed.
Fast rewind back to 1850, and we find in Osage County (Benton township, I believe): Mary (b. KY about 1810), with children named Willliam (b. MO about 1830), John (b. MO about 1836), Charles (b. MO about 1839), and Margaret (b. MO about 1842). Everything points to this being the same family, but I could be wrong.
Osage County wasn't formed (from Gasconade County) until 1841, and I didn't find an Alexander family in Gasconade in 1840. The birth states of William, John, and Charles show that they should have been in MO in 1840.
I would look for a marriage between an Alexander and a Mary (perhaps G. could be for a maiden name) in KY around 1830.