From stories I've read, it seems like a lot of parents don't handle sexual abuse well, especially when both victim and offender are their children. Boob and Michelle aren't unique in that aspect. My main criticism is they've continued to treat Josh with kid gloves since then. Jesus Jail instead of real rehab, financially supporting his family, bailing him out of the Ashley Madison scandal, hiring an extremely high-priced attorney for the CP case, asking people who are casual acquaintances at best to act as Josh's foster parents, etc.
I don't know if it's guilt, denial, self-preservation