Is there anything that he can do lawfully? I am 25 so he no longer has any parental rights over me. His concern is that if he and my mother pass away, anything in his name will automatically be split between me and my two younger brothers and he doesn't want me to have any of it lol
Nope. He has no control over your name. In fact, you could change your first name to his first name if you felt like paying the legal $ to do so. Or even change your name to "The Real [stepfather's name]".
He doesn't sound too bright, because if he's really worried about who inherits, he could just write a will. Seriously, if he adopted you as a minor, no matter what your name is now, if he dies intestate, you could possibly inherit. Lots of women choose to change their names when marrying - doesn't change their ability to inherit from parents who die intestate.
If stepdad DIDN'T adopt you, then it's a little different.
And in ANY case, your mother is your mother.
