I know from experience that the Sienna makes a fine tow vehicle and can handle pretty much any Airstream you want to tow. I've been hauling my 2011 28' International for three years now with my 2011 Sienna and had no issues whatsoever but, and it's a big BUT, you do need to set up properly.
At the risk of getting a concerted groan from my fellow Airstreamers here on the board, I have to mention that my setup was fitted by Can-Am RV in London, Ontario. I'm sure people think I'm on some sort of retainer (I'm not!) but they are the guys to speak to to get full and accurate advice about towing with your Sienna.
Can-Am RV Centre | #1 Airstream Dealer in Customer Satisfaction Worldwide | Your Full-Service RV Dealer
Even if you can't get up here to have work done, Andy and Kirk Thompson are quite free with their advice.
If you want to have a look at my setup, check out my blog - the URL I've listed takes you to an eight-part series that I published about the tow set up:
The Toads of Towed Haul
Good luck with whatever course you take in the Airstream world.