I have friends serving there right now and they know where I stand as to the reasoning behind the war, but they also know I support the troops 100% because they are doing their job.

Personally I would just be honest with these people as they go into their schpeel and stop them mid sentence and simply say, "He's my brother and I support his decision, if you are going to degrade my brother, his choice, and my support of it then you can just go away now before this gets ugly".

You don't have to get rude or anything but just plainly state it as point blank fact and I bet you never have to listen to that shit again.