Having a disagreeable, immature mother-in-law puts a lot of strain on you and your spouse. When she lives with you, that tension multiplies. While she may drive you crazy and leave you feeling hurt, it is important to keep a level head. You are the love of your spouse, but she is his mother. If you push him too hard to stand against his mother in your defense, he may decide that blood is thicker than water. Present all your arguments in a calm and logical manner to make the most impact on both your spouse and mother-in-law.
Unite with your spouse so you have support. It may take a while to bring her over to your side, but if your mother-in-law continues acting in an immature fashion, she is likely to see the behavior in action soon. Don't push her too hard, though, or she may start to see you as the nuisance instead of her mother.
Speak calmly with your mother-in-law about grievances only when your spouse is present and he knows the reasons for your grievance. If she throws a tantrum, he will see her reaction so she can't embellish the encounter later to make you sound like a criminal.
Avoid starting arguments with your mother-in-law even when your spouse is present. Explain that she did something you have issue with and ask her not to do it again. If she continues to perform the unwanted action, it is on her head and no one can point a finger at you for your grievance.
Develop a life outside of your home if her company becomes too unbearable. It may seem unfair that you have to leave your home to maintain peace, but it's better than the constant arguments. Create a schedule where you leave your home to go do something enjoyable, like going to the gym or a book club.
Distance yourself from a situation that is elevating to an angry confrontation between you and your mother-in-law. It may sound crass, but if your mother-in-law is going to act like an immature child craving attention, you need to deny her the attention she wants until she acts in a civil manner. She may try to provoke you, but just withhold your attention and turn to something you enjoy instead of arguing.
Stay firm on matters of genuine important such as privacy, safety and child care. Be consistent on these fronts and do not let her sway your opinion or your spouse's. She needs to understand that your children, money and privacy are yours to manage and she will respect that as long as she lives under your roof.