Thank you, Zlatica and David. I have been helping a friend look for Kostkan family records. They immigrated in 1901, so the village was recorded on the ship's record. I have found random Kostkan records in the various villages in the Zleby books, and Zleby isn't that far away from these two villages. So I alway felt the Kostkans were probably from this area, and Sedletin is not that far away.
Unfortunately, it appears that the records are not yet up on Actapublica. But at least now we have a location!