Hi, there was a brother and sister I think, somehow left behind on a south Pacific area island... Samoan(?) NOT Blue Lagoon..
Sister learned to spear fish and they survived alone on this islands . Made sometime mid 70's

Sounds an awful lot like Island of the Blue Dolphins. The boy does not survive though. One of my favorite books.
Island of the Blue Dolphins (1964) - IMDb

I got your pm Sherilynn - glad it was what you were looking for! I read the book when I was younger, then again out of sheer boredom on a summer Saturday last year. Great story.