There is this annoying kid named sterling with ADD. He will break everything he touches and (if your mom is like my mom) when he break something I punch him.
To prevent breaking I find a place no one looks and hide all my favorite games in there, and never let him out of my sight.
One time when I was at my grandpa's house, My friend asked my mom can she play on my computer, mom said yes, and she played my san andreas. But she is a friend and didnt use/erase my saves.
Here is a strategy, Dont let her near your stuff. make sure she doesnt download anything. And block youtube/other downloading sites.
Hide your DSI
Put your Wii in your parents room for a while.
Put your systems in a closet.
And watch TV/go outside for the entire time she is there.