Because they said so. The police released him after they determined it wasn't a bomb (actually they knew it wasn't a bomb when they arrested him, which they've stated, as well), and after they determined it wasn't a hoax bomb. So after the police officially determined that it was neither a bomb nor a hoax bomb, which school officials were do informed, they school suspended him, anyway, knowing it wasn't a hoax bomb.
You seem to be confusing disciplinary problems with disciplinary action, and assuming the disciplinary action was actually warranted. We already know the disciplinary action of suspending Ahmed was not warranted, and the only information we have on the sister's disciplinary action is that it was based on the say-so of some other middle school student. To draw the conclusion that the sister's suspension was warranted is to do so solely on belief with no facts to support such a conclusion. If anything, the unwarranted suspension of Ahmed points to the sister's suspension also being unwarranted. So to charge that a double standard is being applied to kids with a history of disciplinary problems in school with kids who's questionable disciplinary actions from school officials is retarded. It's grasping at straws trying to deflect and defend.