Thomas Mitchell who married Elizabeth Harmon was the son of Robert Mitchell and Elizabeth Campbell. They were my 3 great grandparents.
I did find that William and suppose to be Abigail Unknown (no records yet on who he married) Harmon had 4 daughters. They were:

Eunice Harmon married to James Mitchell (who was brother of Thomas Mitchell above)
Abagail Eliza Harmon married to Elias Heddon then Robert Scott
Hannah Harmon married to Samuel Heddon (I haven't checked to see if he was related to Elias who married Abagail.)
Then Elizabeth Harmon who married Thomas Mitchell.