You can defeat your brother's access restrictions, either by a timing-attack or side-channel attack.
In a timing-attack, you wait for a sufficient time, your brother will remove the MAC filtering for your device.
If you cannot wait for the time-based attack to succeed, you can use a side channel attack and connect to the internet via an alternative channel, such as GSM or a friendly neighbor.
Joke aside, MAC spoofing is a way to overcome MAC filtering. Since MAC-filtering is (usually) only tied to the MAC-address assigned to a network interface controller, you can change your MAC-address to match the one of an unfiltered device. This is a relatively easy process, but can cause harm (Denial of Service), depending on network equipment and configuration.
On wired networks, switches are usually only designed to forward traffic destined MAC-address to one port. If multiple ports have the same MAC-address, the network logs might contain warnings of MAC-flapping and alert the administrator. This blog post demonstrate how the network can become unreliable for devices that share the same MAC-address.
On wireless networks, sharing the same MAC-address usually do not lead to the same problems as on a wired network. The reason for this is that the wireless network is a single network port (a single radio interface) with multiple connected devices. There are no alternative ports for packets to take as long as both devices are connected to the same access point over WiFi.
Sometimes, you also have to clone the IP-address of an unfiltered device (this is also dependent on the network devices that handle MAC-addresses). This can lead to another set of problems:
- If your network adapter is set to DHCP, you might be issued the same IP-address as your target device.
- You and the target device can get visual warnings about IP-address conflict.
- Your and the target device might drop connections that belong to the other.
If possible, try to use statically configure the adapter to use an unused IP-address. If you absolutely have to also spoof the IP-address, wait for the device to disconnect from the network. There is a tool called CPScam that is used to bypass captive portals (which most commonly use MAC-filtering). This tool will monitor the network for active devices, and alert you whenever a device leaves the network. If you impersonate a device that is no longer on the network, it should not cause harm or alarms, at least not until it reconnects.