"My maternal grandmother and I."
Translation:Shi dóó shimasaní.
You're trying to apply English rules to another language. If you do that to every foreign language you learn, you will always make mistakes learning them. Rules that apply in English don't necessarily apply to other languages. Yes, in English the order "my mom and I" is correct, but this is not how Navajo orders it. They order the Subject "I" first and Object "my maternal grandmother" and then the Verb, if present.