We don't have ROTC at our school, but I did run across a military sorority on the internet. It is the ONLY sorority offered to all branches of the military and they do have a brother fraternity. The sorority is called Sigma Phi Psi and the Fraternity is Omega Delta Sigma. Their purpose seems to be to provide brother and sisterhood to its members as they move from base to base and country to country.
Here is the link to the sorority and you can also click on the fraternity from there.
http://www.sigmaphipsi.org/index1.html