According to information at the Gendex website, both Josiah and his wife Sarah were children of Penelope Castellaw.She was (supposedly) the daughter of first husband, Joseph Johnson.He was (supposedly) the son of third husband, William Dicks.Did Josiah really marry his half-sister?I strongly doubt it.
To complicate things further, he was born in 1780, although the estimated date of the parents wedding was not until 1798, when Penelope was about 42.The period when he and his 7 siblings were being born overlaps with Penelope's first two marriages 9 other children.Has no one else noticed this?
Perhaps William Dicks married a different Penelope Castellaw.There was one who was born about 1775, the child of John Castelaw. She would have been of marrying age in 1798, but then Josiah would have been born too late to marry Sarah Johnson.
Perhaps William Dicks married another Penelope.There is mention in this forum of a marriage between a William Dicks and a Penelope Johnson, but no dates are given.
One contributor to Gendex has the children of William Dicks belonging to an unknown (hypothetical?) first wife, prior to the 1798 marriage to Penelope Castellaw.