you would be contravening the terms of your contract with your ISP if you did this. The connection is specifically for use at your address.
If however your brother was say, at the end of your garden with his laptop, and you wanted hime to have access to your connection that would be OK - and in those circumstances he would need a wireless network card in his computer. You would need a good wireless router, one with a wide range. The ideal model would be a Belkin MIMO Pre-N router.
You mustn't set up a network with your next-door neighbour however, even if he is your brother.