According to the book "Smyth County, Virginia cemeteries; Vol. 01" Sidney McPhatridge died 10 Jun 1860, age 74. Have you been able to locate any death records for her that might include her parents' names?
Also two things you could try:
1. Look for McPhatridge in FamilySearch books. I found several hits in "Protected" books that can't be seen online. You could use WorldCat.org to find the libraries that have those books and try to get them through your library via InterLibrary Loan.
2. Look for Marshall wills/probate records in the areas Sidney lived/could have lived, around the time her father (or possible uncle?) could have died. She could be mentioned. It's possible that these records have been abstracted and published.
Just a couple ideas...