It is hard to understand some people sometimes, who knows what your brother and girlfriend are thinking?!
Like a previous posted said, we always seem to manage what God gives us. As we all look back I am sure we can think of times that we thought we would not get through, but we do, a little scarred yes, but we make it. It even makes us stronger and better people for it I think.
I understand not wanting to be responsible for their mistakes and that is a gut reaction, let it sit for a while, you have a few months to figure things out yet. The fact that your heart can handle another one sort of tells me what will happen though.