I guess I sympathize with MIL's aims, but this is just a huge disaster waiting to happen... and it's hard to believe an adult of sound mind could make a plan like this thinking it stood a decent chance of actually working? Also a headache waiting to happen. If LW's spouse is determined to go along with it, she's not gonna have a lot of options. I mean, she could just tell the rest of the family, but that's probably not going to end well either, although at least it might avoid the headaches that would no doubt fall on her being married to this person as they tried to do anything with this plan.

On another note, though, it DOES sound like the setup of a golden age mystery novel.