Although marriage records show a marriage bond for Mary Ann Alexander and Riley F. Nix in Henry County TN, October 17, 1838, this Mary Ann was unlikely to be Hugh's daughter. According to records from Hugh's family, his daughter Mary Ann, b. January 23, 1824, married Samuel W. Mathis, October 1841, when she was a few months past seventeen. I have seen no other data to show that she had previously married at fourteen.
Sorting out the many Mary Alexanders and Sarah Alexanders around and getting married in Henry County in the late 1830s and early 1840s is difficult, made more so because several of them were probably related. You seem to have some record showing your Mary Ann's birth date. If I obtain any further information on her or Riley Nix, I'll share the data with you if you wish.