I think being honest with your sister should have been the first route. "Hey, I see your son/my nephew is playing video games again, is it OK to buy him a game?" I mean, if the kid's had video games taken away from his due to aggressive tendencies, you should take that into consideration when buying him a present.
There's absolutely nothing wrong with the kid deciding to be honest with his mother. Sure, kids often decide to keep secrets but you have no one to blame but yourself for trying to go a deceptive route. You could have been straightforward with her and saved yourself a big headache. Now you're stuck in a situation you could have easily avoided.
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TheRock ~ Slow dramatic zoom-pan. Doesn't phase the hooded man.
"You have issues." - MWC. Pot. Kettle.