Looking for info on Peter John Cassimatis or Kassimatis

Looking for information on my late grandfather John Peter Cassimatis or Kassimatis. I know he was born in Kythira. I believe in the town of Levadi. He had a brother named George, and two sisters, one named Helen and one Irene.

We believe Helen married Emmanuel Karavonsanos and moved to Athens.

John Peter’s father was Peter John and his mother Irene. He also had a grand father who was a Greek Orthodox priest.