Rachel Kendrick was the daughter of John and Elizabeth Kendrick. John Kendrick died intestate in 1801-2 in Washington County, Virginia; Rachel, wife of William Allen was involved in the settlement of his estate. William Allen and the widow were represented by attorney Elijah Gillenwater. I can't find a William in Washington County in that time period so they must have lived somewhere else. I haven't found when or where William and Rachel were married, though John Kendrick has been traced from Frederick to Pittsylvania to Washington Counties in Virginia.
You mentioned a Mr. Childress in your last message, I'm not familiar with him, where can I find his postings? Lucy Boyd did have a sister Nancy, but I've seen that she was married 29 May 1817 in Patrick County, Virginia to Spencer Cockram. They had at least five children and are found in the 1860 census, still in Patrick County, Spencer is 61, Nancy 65. She must have been ten or 11 years older than Lucy.
There is a Preston Allen in Kanawha County listed in the 1840 census, he lived alone. Can't find him for 1850.