If he is saying things to a third party that are A)untrue, and B)damaging to your charecter, you should immediately file a civil suit. Especially since it is work-related. His comments could lead to you having trouble finding work in the same area or within the same industry. From what you've said, you're father-in-law is slandering you and causing you damage. Collect a list of people he has said these things to and go speak to an attorney.