Is it a good idea to ask my brother to buy me a new laptop?
Why do think he should buy you a new laptop?
Anyway, I already told him about it.
Expecting someone to buy you something is rude. Unless the person made an offer to do so already, they may feel like you're pressuring them into doing something. This could upset them and make them not buy you a laptop.
Ok so I still want it. I'm going to make a big deal about it.
If you're going to be rude about it I doubt your brother will buy a laptop for you. If you're in  desperate need of it you should politely talk to him and see what he says. If he offers you to buy a laptop it's well and good but you should not be make a big deal about it if he says no.