On the other hand… Searching the Highland co. forum, there is information indicating that Isabella Hays remarried and became the third wife of John Hutsonpiller (various spellings) on 20 May 1833, so it would be possible that some of her potential (so far unproven) children by John Hays were still with her in 1840. John Hutsonpiller and his second wife appear to have had two daughters born in the 1820s, but so far they are hard to document.
1830 and 1840 demographics just might have a clue. LDS shows that there are Highland co. listings for John Hutsonpiller.
By 1850, through 1880, Isabella Hutsonpiller is in Pike co., IL. The listings have her as born circa 1793 in PA. And this would be in agreement with the birthplace for the mother of your Rebecca B. Hays.