This is a question that came up often during our class discussions of Ender's Game. We found passages to lead us either way, so our question is, "Were siblings allowed to attend battle school at the same time?" There was no record in the books either way, so we were curious.
I never addressed this in the books, except that inadvertently Nikolai and Julian ("Bean") Delphiki ended up in the same army. I can't think of any reason why such a thing would have been forbidden, at least none that would take precedence over the need of the military to train all possible commanders. So I imagine it would be allowed for siblings to be in Battle School at the same time.