ah i see, it has changed, your first post here was 188.8.131.52
Your brother shouldnt have a problem joining, most likely the problem lies with your router. There is a good guide at www.portforward.com
configuring a static ip will probably be the first order of business
After that you'll have to find the model of your router and the guide will show you how to foward ports for jka.
you can always use http://www.canyouseeme.org/
putting in the jka port of 28070 to see if that port is open. If it's not your router, or it's open and your brother still can't join, then either the problem is your isp, or his college.