I would have thought Mario Kart: Double Dash!! an excellent idea if your Mother isn't all that confident with games. Basically, if you've never played it before, its two guys in one kart. One's on the back throwing all the items while the other is driving. It requires, nay, demands co-operation and a close teamwork to be really good at because you can only do so well with good Driving and your almost definitely going to lose with only a good-strategist on the back. If your Mom isn't as confident enough Gamer to drive, then she can deal out red shell death in the back seat while you drive. She'll still get a good amount of satisfaction of playing a big, proper game without feeling too overwhelmed because you, as the Driver, will be doing most of the hard work. Her role is as vital as yours to win and I'm sure you'll enjoy that. Plus, the Purple Cadillac is pretty sweet looking. Maybe not to drive but still cool.
If she is confident in the ways of the joypad, go ahead and get something like New Super Mario Bros. Its just a fun little thing to knock back with. Yes, its been knocked as Mario Bros 3 with better graphics but its still fun and the co-operative thing is just as important, only much more difficult to pull off. For example, if either of you get stuck and don't feel up to doing a part of a level, another player can pick you up and carry you till your ready to continue. This is risky as both of your characters are now in the hands of one player. The one on top can't do a whole lot to help, giving a greater pressure on your Mario Skills. This is the same if one of the players simply enters a bubble. The whole level has to be restarted if the other guy dies.
Hope that helps. Hope you like Mario games cause that's what I got. I wasn't sure if you were into the Lego games. Batman is my favourite but their all more or less the same. All right time-passers. The above suggestions ought to be much better.