Make them clean the toilet. That will shock them.
Make them sit quietly in separate rooms when they act up. The older one has to stay separated longer. Don't expect them to act older than they are, though.
One of the things you really have to handle with the Wii is that they LEARN that their drama mess when it's time to get off loses them the privilege. Really talk about it with them. Ask them if they want for you to give the Wii away. When they say no, ask them why you shouldn't. Tell them that they have to give you a reason to keep it. If they don't know HOW to tell you to keep it, explain it to them. Help them to learn to say it back to you.
Tell them that in x amount of days, you will give them 1/2 hour to play on it. You will tell them when they hav 5 minutes left. When one minute is left on the clock, say that, and then come turn it off. If the dramatics start, remind them the way they are supposed to act, and if they don't immediately stop the antics, put the Wii away again. You must be 100% consistent. But in order to teach them to do the right thing, you must keep giving them a SHORT opportunity. When they do it right, up the amount of time and let them play some the next day. The FIRST time they backslide, go back to them not getting it.
Remember the army - if one person screws up, everyone pays the price. Employ that here. If one sibling says to the other "Hush - we'll lose it if you fuss!" then you have won. Kids listen to other kids more than they do to their parents.
Good luck!
Dawn