My wife just left me, but changed the password to the admin account on my iMac just to spite me, and I have no way of contacting her. I have a separate managed account for my brother when he was staying with us not too long ago, and she didn't change the password to that. She also signed in with her Apple ID so I can't reset the password, and I don't know her Apple ID password to reset it, nor do I have access to her email. Is there anything I can do to get access to the admin account?
Break a password
search for Mac Password Reset Software
Access user account data
If the fileVault was not enabled then you can access your account data by connecting your disk to another computer.
Contact Apple Support
You are not the first one in this situation. Apple Support should have already an answer for this question