Pictionary is great because being a bit older doesn't help. Me and my 8yo niece make a great pictionary team because we seem to understand each others insane scribbles while everyone just doesn't get it lol.
Also it's played in teams so it makes sibling work together.

I just remembered that there are only 3 of you lol.

Scattergories is fun but you might have the same problem as scrabble etc.

ETA - What about SingStar? Do you have a play station? I geuss there are other karaoke games available that you play through you TV.