There is a lot of uncertainty in the branch of the Avents who moved to TN from Greensville Co., VA, though that's pretty certainly where Sarah and her brother Samuel came from. I've got this reference from a history of Hardeman Co. (though I can't find the name of the source right this minute) that says this:
"SAMUEL TAMLIN AVENT. Born in Hardeman County, Tennessee, on April 7, 1836. Both parents and his brother had died before 1850. The family had connections with Greenville County, Virginia, where he and sister Sarah inherited property in 1854, sharing equally with a kinsman, Tamlin Avent, a citizen of Greenville County who was a nephew of Peter Avent, who also lived in Hardeman County, and may have been Samuel's uncle. He was a master mason and a member of Hatchie Lodge #151 at Middleburg. Died December 19, 1896, and buried in the Avent-Marshall Cemetery."
The lineage of the above referenced Tamlin Avent is pretty clear (and can be found at my website, www.aventfamily.org
) but that of Sarah and her brother Samuel are a little hazy. Hope this helps,