This actually did happen to me, similar situation. I noticed when I was still living with my father that he was stealing my meds, specifically the kinds that are used for my migranes. Now, given that things generally in my household are bizarre and left me with the sensation of "It must be me; I must be crazy" my first inclination was to blame myself. Of course my father couldn't be stealing my meds I reasoned; he would never do that, sink that low. Well, what happened was that, indeed, he had gone to the pharmacy, and told an extremely tall tale to the pharmacist, saying how I had been bedridden with a migrane for an extensive period of time, too weak and in too much pain to go to a doctor, the pharmacy, the hospital, and too stubborn to take his advice; would they please just fill a script for some migrane meds and give it to him, to bring to me, to make me a little more bearable to live with? Imagine my surprise then, not even a week later, when I had a real doozy of a migrane, the kind that actually does leave you bed ridden, and the pharmacist had actually filled the script said "I can't give you anymore. You've already gotten a new pack. Your dad picked it up. I meant to talk to you about that because your file is marked 'restricted access,' but your dad picked up your meds." Ooopsy! After that, the pharmacist made it known only to his associate and to one other staff member that I was no longer enabling my father and, ahem, I handle my own meds, Thanks.
 This also happened with a doctor who happens to also see my father. After this specific incedent, the pharmacist was obliged to call the doctor who over saw the perscription. Of course, I made an appt asap to see this doctor; apparently, this doctor, too, had been duped. My father, great con artist that he is, had persuaded the doctor about the severity of my migranes, the symptoms, et cetera, and some how the meds just flowed right into Dad's hands. Again, notations were made, and ahem, people were informed. 
 And the list has gone on and on. People dad works with. People dad is friends with. People dad hangs around with. Over and over and over again, I've told these stories, related what he's like behind closed doors, even showed them my bruises and bank statements; but no dice. They see the Dad they want to see, and they don't what they don't.
 My point is this: whether the enablers are knowing or unknowing, the key is what is believed. I've finally come to realize that there is great truth in the statement "A length of rope is of great use for two things: pulling oneself out of a hole, and hanging oneself by its length." The reality was that until Dad set himself up his unwitting suppliers didn't believe me; when he was exposed, he was cut off. I think that's the same thing here too, sadly, CG. As much as your MIL knows the truth, as long as she doesn't believe it, it's okay; he's stil her baby. That goes for the rest of the family. And QOD is right, too; given the dangerous history this guy's had with you, if he shows up, call the police. You have a right to feel safe in your house.
 It's okay that you feel like you're losing your mind. It's also okay that you feel like no one believes you and you feel isolated. I'm coming to see, more and more, that it wasn't that anyone didn't believe me at all; in fact, at the meetings dad frequents, people have come up to me at other meetings and said things like "are you okay?" "do you want to talk?" "do you have my number, just in case?" In other words, they know; they get it. They're under no illusions about who dad is.
 I'm glad you shared this with us. Keep us posted.