The Moscow State Treaty, which ended the occupation of Austria and made it an independent country in 1955 forbade Austria becoming a part of Germany and made Austria a neutral country.
In fact, the prohibition on Austria to enter any sort of union with Germany is why they didn't join the EU until 1995, after the Berlin Wall fell. They actually had wanted to join for decades, ever since the EEC was set up, according to Nanwe (who knows quite a lot of EU history), but they didn't because they didn't know whether joining the EU would be seen as a second Anschluss by the Soviets.
That's why they ended up joining EFTA...
So in order to allow for Austria to be included, you have to have the occupation go differently (don't have Moscow State Treaty, at least) and not have the whole "first victim of the Nazis" thing, which played a significant role in establishing a distinct Austrian identity...
Nowadays, Austrians are to Germans what Canadians are to Americans-similar in culture, speak the same language, but consider themselves as distinct peoples and don't want to become part of the other country...