how do i get his pc's IP address?
There's two ways to figure out his IP address:
1) If you have access to his computer, open up a command line prompt. You can do this by pressing Windows key + R. A little window will open. Type in cmd. This will open command prompt.
2) In the command window, type ipconfig. Where it says IPv4, that is the IP address.
If you can't access his computer:
1) open a command window as described above.
2) type in arp -a. This show a list of all IPs on your network. Copy and paste each 'Phyisical Address' in the list (you can highlight in command prompt window and use Ctrl-C to copy) into this website : Arul's utilities.
3) The website will tell you the type of device. Once you find the type of computer your brother uses, you've found the IP you want. If you use the same type computer as your brother, check your IP address, and the one that isn't yours, is his.