HELP with MIL & SO

Let me start off by saying me and my husband have kids together. We have a house, cars under our names. We are both adults. Now my MIL is a cosigner on my husband's account from when he was younger and had gotten his first job. Now years and years later she is still on the account. She always checks to see how much money he has. She always sees where we spend our money. She texted him tonight telling him how much money he has in his account. THIS IS NONE OF HER BUSINESS. Every time I bring this up he doesnt want to create a new account because this account is how he transfers money to his parents. He thinks I'm just jealous of his mother because I want her off the account and I want joint accounts, because we basically share money anyways! He also agreed to a joint account then backed down because of his mom! Ladies PLEASE help me help him see this my way!! How do I get him to take her off his accounts! This is ridiculous. I do not want to argue with him. I want a civil conversation and I want him to see this is not okay under any circumstances. She acts like his wife ALL the time.