Appox.7 days on a Greek Island

Great idea choosing Sifnos. It's still the ideal "postcard" island for discovering the Cyclades, and it's still relatively untouched by mass tourism (though less and less over the years) thanks to the lack of an airport.

The western Cyclades islands, which include Sifnos, are now served by more ferries than 30 years ago, notably by high-speed Seajets catamarans that make the trip in 2.5 hours.

Regarding your concerns about ferry cancellations, there's only one thing to know: there's nothing you can do about it. They happen more frequently in the winter, but it's always a possibility no matter the month of the year.

On the other hand, there are some useful precautions to take:

The first is to return to the mainland 2 days before your return flight. If you return only one day before and the ferry is canceled that day, you're practically guaranteed to miss your flight (even if you take the ferry the next day). As a side note, my first ferry cancellation (due to bad weather) happened while we was on Sifnos, and of course we missed our flight, that was a little over 30 years ago.

Second important piece of info: The earliest ferry departures are in the morning between 7:00 AM and 9:00 AM. Starting in early June, Seajets runs 2 round trips on certain days with a second departure around 4:00 PM. But in my opinion, given that there will be 8 of you arriving from different flights and trying to get to the port of Piraeus, it's going to be very complicated, if not impossible, for everyone to take the same ferry on the exact day your flights arrive.

There is now a metro that goes from the airport to the port of Piraeus in about an hour, and the X96 bus still runs it in about 1 hour and 15 minutes (the airport is no longer in the same place it was 30 years ago). In the best-case scenario, you should count on about 3 hours between your landing time and the ferry's departure time.

I’ve organized island trips for groups of eight arriving from different airports, and I can guarantee that trying to get everyone on the same ferry on the day they land, especially coming from different flights, is something you should forget about.
The best thing to do is to book rooms in Piraeus and leave the next morning on the first ferry. This is what most travelers do when there's any doubt about their arrival time at the port. Piraeus is an interesting place, unfortunately not very well known by rushed North American travelers who prefer to take a connecting flight to an overcrowded island. Speaking of which, I'm sure someone is going to recommend Naxos to you... haha. Don't fall for it!

Being a group of 8, in Sifnos you'd be much better off staying in a house or a large multi-bedroom apartment. You should be able to find something suitable. Check out Rooms in Sifnos, it's the best website for finding accommodation on the island.

https://www.roomsinsifnos.com/

Information on accommodation availability en prices for late May or early June won't be available until around March, at the earliest.

On Sifnos, you can of course rent cars, as well as scooters (subject to certain conditions) and now electric bikes. Otherwise, the bus hub is located at Artemonas-Apollonia, and the bus network is quite good.

Here is an example of the timetable and a map of the bus routes:

https://drive.google.com/file/d/1pGZ5IL7v-SRwF_1tKrqdNCWwB14fMMtX/view?usp=sharing

Agree with @JoLui — both that Sifnos is a wonderful choice and that it is wise to plan for travel delays. We did a family trip to Greece in late May this year. There were four groups coming from NC, CO, TX and NYC. Only one flight arrived on time. One was cancelled entirely so arrival was delayed until the next day. We planned two nights in Athens before setting sail so it worked out, but planning to travel on same day would have been a disaster. Would also suggest that you consider how you will get around on Sifnos as you will definitely want to explore. Having a car(s) or van will make your stay more enjoyable. Have a great trip!

I think arriving in Athens from the island 2 days prior to a return flight home is overkill. Arriving the day before is just fine. It’s what we did 2 years in a row and we didn’t have any issues, although if you’re planning on doing some sightseeing in Athens and the surrounding area then you could always make it 2 or even three days. Sifnos is an excellent choice. It was the first Greek island my wife and I visited back in 2023 (feels like a lifetime ago) and while we only spent 9 days there we feel that we could’ve spent a months there. We stayed in Faros, at Kavos, a small family owned and run hotel overlooking Panagia Chryssopigi and let me tell you that place is spectacular. We booked it through Airbnb, which we no longer use, but still use to search for places.

The idea of coming back 2 days early was only because Searq is concerned about possible ferry cancellations.

Coming back to Athens 1 day before the flight just in case the ferry gets canceled doesn’t really make sense since the ferry schedule back to the mainland don't allow to reach Piraeus before 3 p.m anyway. You’d need to have a flight after 7 p.m. for that to really be worth it.

Otherwise, I agree that it is overkill given the actual risk.