Let your sister buy it, sit back, and enjoy the ride.
You warned her, and it's not your money, so why worry?
Enjoy the carnage, I say.

If your sister is buying things such as Mp3 players for the child, perhaps one of those tough book laptops which can withstand a gunshot is a liable purchase when she turns 12. (Joke)

I'd suggest one of the old school original GBAs. You can probably get one from Ebay for 30-40$. I used to drop that thing on the floor on a regular basis, never really had an issue with it breaking.

If she really must, let her get a GBA. That thing is practically unbreakable. Atleast, unless she can fling it out of a window or something.

You could keep giving good advice about age appropriate gifts. But its probably going to be like banging your head against the wall and you'll just get frustrated. So just sit back and enjoy the I told you so's.