This isn’t a technicality - you don't have a contract with the insurance company, your mother in law does. Your mother in law doesn’t have an insurable interest in the property. You’re fighting an uphill battle that most likely isn’t going to end well. Best of luck to you.
Insurance contracts are not transferrable.
Let this be a lesson for anyone reading this. Details like this matter, it’s not a “technicality”, insurance is one of the most regulated industries in the world.