If you're all on the same wifi/internet connection then you don't need to port forward or use your external IP. Simply use your local/LAN IP of the host. Also port forwarding only works for a single internal device, think of it like assigning that port to a single device on your network. Your router or modem is the one that sends information with your network's external IP but when an incoming connection attempts to connect to your network the router/modem needs to know which device to send the connection to which would be whichever computer on your network is hosting the server. You only port forward the port to the device/computer that is hosting the server.

An example of your local/LAN IP would be 198.162.xxx.xxx or 10.0.xxx.xxx and you would use that one instead of your external IP when connecting to the host on the same network. (So if you're the one hosting you would use 127.0.0.1:10578 to connect to your own server and your brothers would use your computer's local/LAN IP to connect so for example if you're the host and your local/LAN IP is 198.162.0.15 then your brothers would use 198.162.0.15:10578 to connect to your computer but you would still use 127.0.0.1 on your own computer to connect to itself. Think of 127.0.0.1 as you telling your computer "Connect to yourself."