This may sound completely crazy, but trying to rule out as many things as I can.

Using your same login credentials, can you try logging onto the SE website at another house / location (as long as it doesn't use your current router-ISP combination). If you're using a smartphone to test this, do not use the WiFi to connect to the internet.

IF SE lets you log in from another house/network, going a step further... if you have a spare router in the house and swap it out with the one that is currently supporting yours and your brother's connection. If no spare router, a little further would be to plug your desktop directly into your ISP box and see if you can login to the SE website and/or the game.

If both are true, there *could* be something (not 100%) with your router that's restricting access to SE. Where there may be a fix using the router, without reproducing the issue myself, I don't think I could provide instructions on fixing it short of buying a new router.