Sorry you’re dealing with this. That’s definitely not fair to you to force a signature at 18 years old. You definitely need to talk to a lawyer and to your mother.

One thing to consider, as others have said, is that you might not have to be his caretaker in the way that your mother is. She might just be trying to ensure that someone who genuinely loves him and knows him is making the decisions about what happens to him. She wants to make sure he has an advocate and doesn’t get lost in a system. That’s still obviously a big decision for you and lots of responsibility (and possibly financial commitment) but you might not need to be afraid that you will become a 24/7 nurse.