It would have been similar to "//write $decode(rubbish)". Your brother should not have access to your client if he doesn't know the basic rules of IRC. Never type what other people tell you to.
The $decode "virus" is fairly benign, it often gets you banned from channels or networks because it makes you send the infection message to other people. Type /!remote off, go to ALT+R and unload all scripts. For more specific instructions, you can go to Undernet's #dmsetup or #vh channels.
Please note that mIRC is not a file trading program.