If get a decent signal from your brother’s Wi-Fi network and he does not mind sharing his Internet connection when ever you need to link to his network, the simplest method would be to get a Wi-Fi adapter for your PC and connect it to his network.
Generally if Windows sees two sources of Internet connection, it will use the connection that establishes first and usually the wired connection is the one that connects first. So even if you continue using your wired connection and a separate Wi-Fi adapter to connect to his network, it’s likely your PC will continue using your wired connection for Internet access.
If your main intention is to use his network for Internet access and you need to connect multiple devices to it (e.g. phone, tablet, etc.), you will need a wireless access point unit in addition to your router, assuming your router has an Ethernet WAN port. Most Wi-Fi routers cannot use the built-in Wi-Fi for linking to another network.
Once you get a wireless access point, basically what you need to do is configure the access point as a wireless client and connect it to his network. With most access points, this process is fairly straight forward, i.e. when it lists the available networks, you choose your brother’s from the list, then enter his Wi-Fi’s key and confirm it shows ‘Connected’. Then connect that access point to your router’s WAN port with a network cable. If all goes well, you should be able to access the Internet. I would also recommend getting a directional antenna for the access point, as this way you can aim it at your brother’s place for a better signal and more solid connection.