Can she give me joint custody without going to court using a notarized paper?

My mom has had custodial custody of my nephews since one was 2 weeks and other was 6 months. They are 5 and 6 years old now. I have been helping her since she got them (she asked) and everything pertaining to them I do doctor’s appointment, school, and she helps me financially with them if I can't get it right away. She wants to give me joint custody of them so that the things (like seeing a child therapist). She would have to be present for I wouldn't need her.