This was a library book I read in the 1960s. It might have “White Horse” in the title. Cover had a painting of an abstract white horse running on a beach. It was about 3 (or maybe 4?) very different sisters spending a summer somewhere away from home. Different chapters were from each of their points of view. I think there may have been a mother also. I remember that one of the sisters built a very elaborate dollhouse in the attic with lots of very detailed tiles, plants etc. Another sister had something to do with the horse, either real or imagined.
Lost Classics of Teen Lit: Name That Book! 