Young adult book published from 1950 to mid 70’s; there are sisters and I think one of the sisters is named Regina and quinine is used in the book as a treatment or prevention of something. It may have taken place on an island but that part I’m not sure about.
Possibly Natalie Savage Carlson’s ‘The Half Sisters’? One of the sisters is certainly Regina. It isn’t set on an island; but the same author wrote ‘Jean Claude’s Island’ – is it possible that you are amalgamating the two books?
Maybe Baby Island, by Carol Ryrie Brink? I think at one point, the younger sister, Jean, uses quinine as a cure…I think on the pet monkey. I believe one of the babies is named Regina, or something similar. The older sister is Mary…Lou? Mary-Ann? Mary-something.
I don’t believe this is Baby Island. I just re-read it and there is no use of quinine and there is no character named Regina.
Just to clarify the type of “sister” you are looking for: Are you looking for a book about a nun named Sister Regina, or about sisters (siblings) one of whom is named Regina?
Because quinine is used to treat malaria, it sounds like you might be looking for a book about nuns treating malaria in some tropical region? Possibly they were running a hospital or working as missionaries?
I’m looking for siblings, not nuns, and one of them was named Regina. I don’t remember if the quinine was being used to malaria. I’ve wracked my brains to come up with more identifying info and I just can’t think of any.