Weird Computer Sharing Problem - Help Please

Me and my brother have one computer each, and they are linked by the internet (my computer got the modem and the rooter and my brother´s recieves it´s signal). We are having a very weird problem nowadays: everything I write here on my computer appears in his computer. On the past the problem was the opposite: everything he wrotes there appeared here. What´s the problem? How can I fix it? Ps: not only what´s written on one computer affects the other. Mouse clicks in my computer can open programs and folders in his computer.

Hi, Sorry to hear of the problems with your computers. Are you using wireless equipment on your computer (wireless keyboard/mouse) or wired varieties? Regards paws