WiiGames is a good choice.
Mario Party is aswell... maybe when you two are away from the impatient brothers, you may give another shot.
Maybe this is too hard, but, how about super paper mario? Does not need too much controls at all.
If you own a N64, try to get Yoshi's Story, she will love it. The easiest stages are:
I did that selection for my wife hehe. Just tell your mom to make sure to get at least a couple of huge hearths in 4-1 (if not, she won't be able to go to 5-3) Also on 2-1, one is enough and you find it very easily (there is poochy indicating its location... everybody knows that if a dog smells and bark in one place, there is something.