I'm going to be the lone dissenter. And take this with a grain of salt because you know your family and how they feel about surprises. Some people do love them, I'm just not one of them.
And here's why. If my son were going to be home from overseas service, and I'm assuming he'll be home for at least a week, I'd really want to know in advance. I'd want to make plans for the time he was going to be here, and by the same token, not make plans that didn't include him, which I might do if I didn't know he was coming. Plus, for me, the anticipation of him coming home would be part of the excitement.
Christmas is one of those holidays that is quite a lot about the anticipation. And going through all of December with the thought in mind that my son wasn't going to be there to share it with me, when I could have been thinking all month about how great it was going to be to be able to spend the holiday with him, well, I'd just rather have known.
Now, if your only concerns about this is the gift issue, and you feel that your family would be more thrilled at the surprise aspect than if they knew beforehand, then by all means, I would not let the gift thing stand in my way. And I agree with a PP that if they are ok with the surprise, your going to Starbucks and returning with your brother would be pretty cool.
So for me, the only question is, do your parents love surprises?
ETA: Regarding Cat-Fu's question:
How could one feel unpleasant things when their beloved child/nephew/grandchild is with them instead of in a war zone?
It's not that I'd feel unpleasant things, not at all! Just that I'd rather have known in advance for the reasons stated above. Of course I'd still be thrilled to see him!