Yes, you could do away with your cable tv altogether but I would suggest paying for an extra cable box at your brothers, especially if you can get one with DVR abilities.
Yes, it will affect your brother's internet usage as you are streaming the signal from his place. I have my Slingbox at my place in the US and watch it at my home here in Japan. My ISP in the US allows 250 Gb per month and I just about use it up. I stream in HD from my Slingbox Pro HD and I have high speed fiber optic broadband internet service here in Japan.
Hope this answers yoru questions!