Germany has been taking in a large number of migrants. However practically speaking any and every country has a limit and a certain capacity to help. The migrant crisis is already changing the statistics of most western countries and is affecting the social and political setup of most countries in a very big way which may even lead to the collapse of the EU.
Countries should be doing anything possible to accept and find safe lodging for immigrants running from war torn lands. The number one priority of any country should be to protect human life. This should not just be restricted to residents of the country in question but all people in need, and fearing for their life.
Most of those fleeing Syria are not migrants, but refugees fleeing a homeland decimated by civil war and the incursions of ISIS. It is in the best interest of Germany and Austria to cooperate in sheltering these people because of the moral and economic imperatives of the situation. Sending the problem elsewhere will only make it worse because ISIS uses perceived anti-Muslim actions as propaganda. Austria and Germany have an opportunity to prove to ISIS and other anti-Western forces that Europe will not turn its back on Syrian families in need.
Germany is not justified in sending Syrian migrants back to Austria because it needs to take some responsibly in the current migrant immigration crisis. Germany is the largest economic power in the European Union so it's contribution to helping solve this crises should reflect that. Also, there is so much more room for migrant resettlement in Germany than in Austria.