No wonder this is in Sifnos.
The chapel of the Seven Martyrs is one of Sifnos's 237 churches. It stands on a small hill, overlooking the Aegean Sea, right next to the picturesque town of Kastro. It is definitely worth a visit if you combine with a visit to Kastro, where you can enjoy a drink at the "Kouvanos" Kavos Sunrise bar (you'll know when you've reached it because all you'll be seeing around will be Jose Cuervo bottles), or at the Loggia and then try some fine dining at Cantina nearby.
Tip: if you're there for sunrise, you'll see the sun rise from the horizon and that's truly magical.