Burl, I am sorry that you are going through this, and reiterate what nearly every one on this board has said.
It's a temporary problem, unless you somehow get tagged for Domestic Violence. So keep yourself out of situations where you can be attacked and defend yourself, since you know that your unhinged wife and family will be looking to provoke a response from you and use it against you. You need to be sure that your divorce attorney is giving you the very best advice. That means that if you don't have a lawyer, you find a specialist in domestic relations and listen carefully to his counsel. Keep a bound book or other journal that describes any misconduct, threats, or misrepresentation made by your spouse, her friends or family in chronological order.
If your lawyer says report the threat by the FIL, then do so in as matter of fact a way that you can. Show local law enforcement that you are a calm and reasonable person, and if your wife or FIL is volatile and reacts when contacted by the police even better for you. Show up at the station in a pair of slacks and an sport shirt, make sure that the police view you as a good guy. Enough of them have been through this, that they will empathize with you if you show that you are more like them and the wife is not a sympathetic figure.
Expect that the kids will report everything that happens at your house, from second hand smoke to an occasional beer you might drink. Expect that your ex will attempt to magnify your conduct, and trivialize her shortcomings. So don't give her anything to hang her hat on or mischaracterize to the court.
Good luck man. This will pass.