Mental Case Brother In Law Will Need To Be Evicted.

My wife's brother has been living in their father's spare bedroom of his condo for about 4 years. He lives there rent free and there is no lease. This is in St. Louis, MO county. He has always been an asshole and has recently become aggressive towards my wife. He's 45 years old and while living with his father has almost no conversation with him. We believe the father is scared of him. The BiL has a gun in his room and mental issues.

My father in law recently had a fall and while my wife was over to help/clean moved an item most of us would call trash to the storage room. This resulted in a text from her brother calling her a cunt. We know at some time in the not to distant future he will pass. My wife is the executor of the trust. I am anticipating that we will have to evict the brother in law.

Can anyone tell me what that will look like legally or advise what steps we should be trying to take before the FIL passes? I believe he will be reluctant to make him sign any paperwork at this stage.