A text indicates "I am the church of the most august royal couple Constantine and Helen. In the times of sultan Ahmed I was entirely built (rebuilt?). In times of Abdul Mejit I was adorned as befits me and in the era when the renowned Paisios was bishop through efforts and expenses by the people of Sinasos erected from its foundations in 1729. Repaired in 1850."
So now you know as much as I do. I think the church was founded in ancient times, then destroyed and rebuilt, maybe a number of times. I guess it has been in use in the early 20th century, it now, seemingly slightly restored, can be visited as a kind of museum. It's in the heart of town.