Your advice (and I use the term loosely) should come with the number of a divorce attorney, because the net result would be that she’d take the kids and be gone in the morning. FIL would enjoy a good chuckle, when the poor guy had to come to his house with his tail between his legs to pick his kids up every other weekend. Dear Abby you ain’t.
My “advice” is base on experience. But make no mistakes, if one is going to go that route it cannot be done with out the consent of your wife.
This is what I did....I explained to my wife what her father was doing. And that it was not his place to insert himself and his opinions into our household. I made it clear to her that if her father were successful with this disowning me over the issue at hand, that divorce was our future as the logical conclusion of his activities. That it was about power.
The key here is to do things calmly...explain where they will inevitably lead and did she want to go there. When presented to my wife calmly she understood. She was not happy about my methods but understood that I had no other leverage with my father-in-law and that I needed to assert my authority.
I sucked up big time to my mother in law, but she too understood why I did what I did and agreed that if I acquiesced it would have only gotten worse.