I apologize to you and any others who have read that Thomas and Sarah were sister and brother, as I may have been the one who posted this maybe 15 years ago, when I first started doing genealogical research.Shortly thereafter I realized that this could not be true as wills were obtained and the dates of Sarah Blundon/Blunden/Blundell Thompson did not fall into the category for any of our Thomas Blundells.Also Fairfax County did not fit in with our Thomases except for a possible later one.I, too, have tried to find out who this Sarah was and who she belonged to, but have not yet been successful. Vincent and Henry Harrison, her children, are the only clues that I can think of...naming patterns sometimes help. There is a Thomas Blundell IV b 1725 Westmoreland Co who married a Jemima possibly Harding/Hardin abt 1743-1753 that may have gone to Fairfax but we have never proven if they had children.Fairfax records are sparse in some instances. I bellieve there is now a site wherein you can get chancery records for Fairfax online, but there is a fee.Perhaps there is a clue in those records. In 1820 Fairfax Daniel and Sarah Thompson had as neighbors Michael Robey and Coatney(Courtney) Kidwell and they are in our family.Michael Robey is also mentioned in a court record of John Blundon who died abt Mar 1824 Fairfax, but that John is tied into an Elijah Blundon of Westmoreland Co.Elijah had a Sarah b circa 1772 and she married a Horseman.Another Sarah was a sister of Thomas Blundell IV, but she married a White/Wright/Waite (we've never found much on her except her accounting as an orphan when her father died.
Good luck with your research, is all I can offer. If we all keep looking, one day something may come to light!