This sounds like the kind of situation where in this small town your husband's family is so prominent they have streets, buildings, etc named for them, and I would not be at all surprised if you replied that this is true (but please don't as we do not want anyone to be able to identify you, particularly since these people do sound unbalanced at best).
Maybe your parents can take you and your son in as well as your daughter at least short term. If nothing else I hope they can help you settle as SOT said, somewhere away from the influence of this family.
It makes perfect sense to me now as well that so much has been swept under the rug. I would not be surprised to hear that your stepson's counselors, and everyone else interacting with him, is somehow beholden to your great grandparents in law in some form or fashion. Nobody wants to knock the engine off the gravy train, risk their own livelihoods or be driven out of town on a rail, all of which from what you've described your in laws would be quite willing to do if they felt "crossed" by anyone.
I hope that your marriage will ultimately be OK. I have learned that the biological parent, and this does make sense, is always slower to realize hard truths about their children/much more willing to make excuses and minimize. My wife is much more clear headed about her children since YS' suicide attempt in early September. Unfortunately it doesn't help her relationship with them, though it has made things between the two of us easier.
Blessings, hugs and prayers to you and to yours.