From the way it sounds, you are the one that wants a Mario game, probably for when you visit her.
If your aunt is anything like my mom, get a 2D Mario game, get some of the classics off virtual console.
My mom, as soon as games went to 3D worlds, she stopped playing. Her complaint was that new games now feel like a maze, and she gets lost.
I never got it, she can navigate real life, and can remember directions fully maybe once or twice after she has been somewhere, but a 3D world in a game is a maze to her
I have the first Mario Galaxy game, it was pretty fun, though I haven't picked it up since I beat it, not even to complete getting all the items and things.
Since it sounds like your aunt is really new to gaming, I wouldn't get one of the Galaxy games, or a beginner they are crazy. I don't really know how to describe it, but Galaxy is more 3D than any 3D game.