Yeah, um, been there, done that, got the T-shirt that says "don't poke the crazy", went on high blood pressure meds and lived to tell the story.
For me it was my mil, after my FIL died, she told so many lies about all three of her kids, to people that we loved and loved us. Truth is, she has a mental illness that causes her to do these things. She has finally offended everyone she knows and lives with her brother who had her sign over the will and everything she owns to him.
She has Borderline personality disorder. Look up the symptoms on BPDfamily.com. See if it fits your FIL. If it does fit him, the best you can do is avoid him and protect your family. There are great workshops and lessons on boundaries on that site. People with this problem tend to find someone to paint as the bad guy. In their minds, they could not handle the reality that they have offended their son so bad he won't talk to him, so he must find someone else to blame, enter you.
I could be totally wrong. Your call.