...I've just looked at the public trees on Ancestry & found someone has already researched this family
Of course this all has to be double checked;
Isaac's father was John Austen (1758-1820) born Brenchley, Kent. He married Sarah Martin, 1791, Sussex, Delaware, USA.
... Jane
Checked this marriage;
Although the parish is not shown, there is a chance this marriage might have taken place in Sussex as that was where Sarah Marten was from, so there might be more information in the Sussex Marriage Index.
Pallot's Marriage Index for England: 1780 - 1837
Name: John Austen
Residence: of Brenchley co Kent
Comment: Carpenter, bachr, act 32
Spouse: Sarah Martin
Spouse's Residence: of Wadhurst
Spouse Comments: act 21; spinster.
Marriage Date: 1791
Parish: Sussex