Make sure the Wi-Fi router is not banning you. Wi-Fi routers have a setting to allow/deny specific MAC Addresses which could be used by your brother (to tease you).
If you are allowed to connect with router, this means DHCP is not working for you. DHCP service automatically assign IPs to devices in network. DHCP server on router could be corrected after reboot. It has assigned IP to your brother's device, after all.
If rebooting router doesn't work, it means that DHCP client in your device is not working properly to obtain IP. Try turning off - on Wi-Fi, restart device. If its not a big matter, you may do factory reset, too.
If it's also not working or you don't want to do factory reset, Fix IP of device. On Wi-Fi settings screen, hit menu button and tap
Advanced. From here, you can fix IP to device after ticking
Use Static IP address. If you don't know IP addressing, grab your brother's IP and change digit after last dot. Make sure, no other device in network has that IP.