Turkey, Greece and Venice

That's a lot of moving around. Remember that when you change cities, you lose at least a half day, and with some of your changes, more like a full day. The day of departure from the US and the day of departure from Europe are not days on the ground in Europe, and the first day of arrival is often a jet-lagged haze (particularly with the time difference and travel time from the West Coast). I didn't look at a calender, but I think you're counting travel days as sightseeing days. If you leave Istanbul on day 4, you don't then have day 4 for Santorini (only a half-day at most). Re-do your itinerary, and think in terms of nights, and don't double-count days. Then see what you really have, and work from there. The only possible sensible ferry in your itinerary is Santorini/Mykynos to Athens. I'm not sure how long these ferries take, so it may still be better to fly. The other city pairs are all much too far apart on your short time frame - you'll have to fly. There are no trains connecting your city pairs, and taking boats would require 1-2 days per route (you'd have to get from your departure city to a port city to the overnight boat to your port city to your destination city...) continued..

continued.. "are the days sufficient for what we plan for each destination?" You didn't say what you plan for each destination. But if your plan is "relax, slow pace", then NO. You have time to relax on the islands, as there are few "turnstile" attractions. Even there, I'm not sure I'd try to see both islands (I haven't been to Greece). Your three cities have LOTS to see, and Athens and Istanbul are huge and can be tiring. I'd cut one destination, and have enough time to enjoy the rest. Also, if your goal is to save money, remember that flights cost (not just the flights itself, but getting to and from the airport). You'll save by seeing fewer places and staying a bit longer in each.

I agree with Harold- if you want to a slow pace, this isn't it!
I'd axe one destination. Istanbul is a fabulous city, and if you hustle, you can see a lot in 3 days. The RS book will be really valuable to you.

Kim, 7 days might be a bit much for Santorini/Mykonos unless your aim is to really just relax. If you do drop a day there consider adding it to Venice. TC

Another big thing to remember (which applies unless one or more of the adults is a Drill Sergeant who can lay down the Law and make it stick!) is that all the teenagers I know nowadays don't like to get up much before noon.... so unless you are willing to leave them behind at the hotel you waste half a day at each destination -- and getting underway for an early flight or a ferry is a tiring intergenerational struggle.

7 days split between Santorini and Mykonos is definitely NOT too much. A week will fly by, especially since you'll lose much of one day in transit, starting with checking out of your hotel, making your way to the ferry port, taking the ferry to Mykonos, finding your hotel and settling in. If you want hotel recommendations you need to tell us what your maximum daily room budget will be for the 9 of you. Also, try using www.booking.com to hunt for hotels that fit your needs. There is a filter system that will help you narrow down the options to what you want for yourselves. There are also lots of reviews. You can also go to www.tripadvisor.com for more reviews and to contact the reviewers by private message to ask more questions you might want answered. If you want to rent a private villa large enough for all of you try http://www.vrbo.com/search/keywords:santorini

Thank you everyone for your comments and advices. We will revised the itinerary without Istanbul for sure and also breakdown the days down to travel time details. Once we complete revision, we will ask for your help again in term of logistic between destination, lodging, where to eat and attractions. Thanks again for your help.
KT