There definitely was a third daughter born to John and Margaret between 1790 and 1800. Given the birth years of Robert, John and George (1796, 1798, 1800, respectively), it would stand to reason that the girls were born in the first half of the decade. We know that Amy Jane was born in April 1793, and it appears that another sister was named Peggy (Margaret) and she was most likely born in 1790 based on the Lancaster co SC census.

So, when was the other sister born? She could have been born anywhere from 1791 to 1795 actually. But there was a woman named Isabella Price who lived next to John and Margaret in 1840 Union Co Ga, and was still there in 1850 and 1860. She is buried at Bethlehem Baptist Church cemetery right next to William Harden Patterson (b 1832), my gg-grandfather, and who I believe was her nephew.

Isabella was born 10 April 1792, what would be one week short of one full year earlier than Amy Jane was born. It is also possible that Isabella was a daughter of Robert and Elizabeth Patterson, but given the extremely close proximity of Isabella to the Pattersons in Union Co Ga and also her burial location, I would lean toward the theory that she was the other daughter of John and Margaret. I'm not sure how to explain her being left off Mary Patterson Fountain's 1908 list, but those branches of the family that had moved away were quite lacking from her list, so that may be the reason.

That said, I must make it clear that I cannot prove this Isabella was a daughter of John and Margaret. This is purely an educated guess for now.