My Sister Sif Book

I read somewhere this was a dystopian novel. It was with a few surprises. Written in 1986 it was an early predictor of climate change with a focus on the impact to marine life. So the book is set in an idyllic Pacific island no one has heard of, with whales, dolphins, fish and merpeople. The narrator is a 14 year old who we soon find out has been born of a land person and a merperson.
She and her sister Sif go through various good times and bad times on the land and in the seas. The narrator experiences the pangs of puberty where adults are seen as another species and behaviours are erratic. While the book is clearly focused on its environmental messages its handling of the confusion of growing out of childhood was another highlight.
It remains very readable although the author is probably turning in her grave that her message on the dangers of greed and consumerism has gone unheeded.