when i adopted my boy (just under 10mo) he was very sweet, gentle and timid the first 2 days. he got along with my older female immediately and even a couple of the neighbors dogs. after that for about 2 weeks he was uncontrolable and would bark, lunge, and raise his hackles at other dogs... a few shepherd people that i talked to suggested that i was doing too much too soon and the fact that he didnt know me or his new environment - he had no trust. so what appears as an offensive reaction, is actually their defensiveness against the world until they are more sure of themselves.
i started from scratch by hand feeding him all of his meals for the first month, no intense training or punishments. i walked him during times that we were least likely to run into all the neighbors and their dogs, and if we did, i'd just let him observe the other dogs from a distance, then gradually work from there.
i noticed that he got alot of his confidence from my older female and during times that i'd walk him alone he would revert back to his old behavior, so in your situation, it could be the separation from his sister is also throwing him for a loop.