Josh Duggar's Sister Jill and Derick Say Child Porn Prison Sentence is God's Vengeance

Josh Duggar was just slammed by his sister and brother-in-law ... calling his prison sentence for child porn God's vengeance.

Jill and Derick Dillard posted, "The Bible clearly states that God effects justice and vengeance through the governing authorities. Though some believe Josh should have received a greater sentence and still fewer believe he should have received a lighter sentence, God has carried out his vengeance today for his unspeakable criminal activity."

Jill and Derick says 12 years, 7 months behind bars and 20-years probation thereafter is the only way Josh can rehabilitate himself, "to the point he can safely live in society again."

They went on ... "Hopefully, Josh can actually begin to get treatment and begin to work toward a lifestyle where he is less likely to reoffend. If for nothing else, the notoriety of this case has hopefully contributed to the deterrence of potential offenders and will help protect children by decreasing the demand for' child pornography."

Josh's lawyer, Travis Story, tells TMZ they will fight the conviction on appeal.