I think you are focusing on the wrong thing. I spent 15 years trying to manage a budget with my xah, including hiding checkbooks and cards, separating accounts, leaving my purse at work, and dozens of different budget plans and strategies. It does not work. He always found a way around it. He sold things ( like his vehicle ! ) bartered things, took loans from family promising tax returns, raided his 8yo's piggy bank, and more. It is like staving off the ocean with a tea cup.
Don't waist your time wringing your hands over your MIL. She is not the problem.