Why its awesome being an only child ^w^!
But onto your little problem.
Wait a bit for your folks to calm down and when you're parents are resting up try to go up and talk to your dad. First thing is to say that your sis can have the Wii for now and that you really don't care about that one incident. What you DO want to tell him is how you don't like it that she never has the decency to return it to your room once she's done with it because it is your Wii. You don't mind sharing it and she can take it when she wants but you just don't like it that she doesn't give it back as if its hers.
Hopefully your folks will be calm enough to take you seriously and talk to your sis when she's done and hopefully from then on you can take proper ownership of your Wii again. If she doesn't return it again in the future tell you parents, just don't do it in a whiny-bitchy voice like kids on TV cause that'll just piss off your parents. Take everything seriously, calmly, and at the right time and hopefully everything will be easier.
Magnet-Boy
fuck your sister and your parents
MetaKnightPWNS4
EW!