After sorting through a lot of confusing records, I think the father of Austin and Benjamin F Pettit and husband of Nancy is Thomas Pettit, son of Nathan Pettit 1775 of Spartanburg Co., SC. Thomas was born in 1800.
It looks like Thomas Pettit moved to Butler Co., OH between 1828 and 1830. (His father Nathan also moved in this time frame to IL)
Thomas was living in Butler Co., OH in 1830 when the census was taken. The 1830 census shows only one child, a male, under age 5 which would probably be Austin.
Thomas probably died sometime before 1837 when Nancy Pettit remairred James Vanhorn in Butler Co., OH.
I am not sure what became of James Vanhorn but by 1850 he was out of the picture.
Nancy Vanhorn was found in Butler Co., OH in 1850 with Austin Pettit in her home.
Benjamin F. Pettit was in Butler Co., in 1850 in the home of Dr. Fithian.
Nancy, Austin and Benjamin F moved to Edgar IL by the time of the 1860 census.
The 1858 chancery suit involving the heirs of Nathan Pettit said that they were all living in IL at the time the case was advertised in the newspaper. The suit lists Austin Pettit as the heir of Thomas Pettit who was deceased. Other papers in Nathan Pettit's will box indicate Thomas Pettit left "one or two heirs in Ohio". This was probably before Austin and Benjamin F Pettit moved from Ohio to IL.
The Benjamin Pettit and Nancy Vanhorn who married in 1825 in Lewis Co., KY is probably unrelated. This Benjamin Pettit was in Green township, OH in the 1830 census. They seemed to stay in Green Township, OH and they're probably a different group. Green township is across the border from Lewis Co., KY where he was married.
This means Nathan Pettit's line is NOT extinct as previously thought and I would be VERY interested in finding a male Pettit from both the Austin and Benjamin F lines to do a Y-DNA test so we can compare them to the other sons of Joshua and Rachel.