First go open command prompt and put in shutdown -i and hit enter your gonna need your brothers lan ip for this to work.

Now where it says computers click add and then add his lan ip address and click ok now under comment put anything you want
and when you restart or shutdown his pc the message you put will pop up on his screen

and if you wanna annoy the sh** out of him put display warning to 60000 enjoy

I'm not responsible for what you do to your bros pc with this.

Your brother has to be on your network for this to work and you must have his lan ip