The Islanders by John Rowe Townsend

The inhabitants of Halcyon Island are ruled by the laws of the Deliverer and follow the same customs and traditions that have prevailed for hundreds of years. The laws state 'No incomers', but when Thomas and Molly find a shipwrecked canoe with a boy and girl, barely alive, inside they are determined not to reject them outright. New blood on the island is bound to cause a disturbance, but the effects the incomers have on the close-knit community are beyond anyone's predictions.