I have 2 ideas to help you out here. I did both and they both worked for me. My parents used to be the same. I'm 13 too and they just don't care anymore.
The first one is and isn't sneaky at the same time. Convince your mom to let you get a gamefly account. Since you will know the password this will work. Make sure she lets you set up the account though. She will block m rated games with the same password as the login password. Go for a few months with just e and t games. Then once she trusts you enough, start renting m rated games without telling her. Just be really careful though.
The second one is to just look at the game you want on youtube with your mom. If she sees the gameplay before you buy it, she might think about letting you have it. Good luck.