I remember reading a young adult fiction book about 20 years ago about 3 sisters who were kept isolated and homeschooled by their dad. They were named after the Bronte sisters and their education was modelled on their life.
They definitely lived near the sea.
Then along comes a boy called Christopher, who befriends the eldest (she calls him Kit) and they fall in love and ultimately she decides she wants to be part of the modern world and go to school.
There's various details I remember like how the family doesn't have a "Branwell" son and her father says that's because all boys would ultimately turn out disappointments like the real Branwell 
And one night Kit and the girl get locked in a church overnight and she then tells her family she's pregnant because all she's been told is babies come when "a man and a woman spend the night in the sight of god" 
I would love to find it but googling is only bringing up the real Bronte sisters!