Thirteen Days by Sunset Beach — Ramsey Campbell

It's Ray's and Sandra's first family holiday in Greece, on the island of Vasilema. The skies are cloudier than anywhere else in Greece, and they're intrigued by local eccentricities―the lack of mirrors, the outsize beach umbrellas, the saint's day celebrated with an odd nocturnal ritual. Why are there islanders who seem to follow the family wherever they go? Why do Sandra and the teenage grandchildren have strangely similar dreams? Has Sandra been granted a wish she didn't know she made? Before their holiday is over, some of the family may learn too much about the secret that keeps the island alive.