Ham, you now have the findings on the insurance company's experts. Who are arguably objective experts, since anything they find is to their (the insurance company's) own loss. And if THEY used the Fraud word...try to get that in writing, i.e. ask casually "Can you send me a memo or a letter saying just what you told me, for my files?".
Then get your boat out of that yard ASAP and...
Ask the father in law to meet with you and the county DA, whose job is to prosecute criminal fraud. If I'm not mistaken, a finding of civil or criminal fraud (and you can bring both cases, the F-I-L can fill you in) would result in what the lawyers call "treble damages", i.e. 3x the sum of all of your damages, direct and inconsequential combined.
So there is very good reason to file suit, especially since you'd start with the criminal suit which costs you nothing. If the insurance company experts have said it looks like fraud--that's probably enough for the DA to file on.