Actually as far as the US government is concerned, your brother is not only a citizen, but a Natural Born one. This is because your parents were serving the country, and had not taken up residence in Germany. The law is very explicit that anyone born to two US citizen parents is a US citizen, but Vattel's "Law of Nations" has an explicit exemption to the usual "Born in the county of citizen parents" requirement to be Natural Born, for children of citizens born outside a country but "in the armies of the country". It's the same situation as McCain, even if your brother was not born on a US installation.
8 USC 1401:
; (c) a person born outside of the United States and its outlying possessions of parents both of whom are citizens of the United States and one of whom has had a residence in the United States or one of its outlying possessions, prior to the birth of such person;
Law of Nations
§ 217. Children born in the armies of the state.
For the same reasons also, children born out of the country, in the armies of the state, or in the house of its minister at a foreign court, are reputed born in the country; for a citizen who is absent with his family, on the service of the state, but still dependent on it, and subject to its jurisdiction, cannot be considered as having quitted its territory.
The Germans may of course have other notions, but it is my understanding that in order to be a German Citizen, one must have German citizen parents, and that it's almost impossible to become a naturalized German citizen under most circumstances.
I appreciate the clarifications fellas, but be sure, I watched as my father, sitting in Ft. Eustis, filled out that waiver exempting my brother from being drafted into the German military.