I discovered your website when googling several key words of the book I am seeking. Someone else had also looked for it but I couldn’t find your answer to them.
The book is about four orphaned siblings who move from Britain to Australia as their guardian is an unknown great-uncle. The young people are Alice, Betty and Sigismund plus another brother whose name I can’t recall. Due to a case of mistaken identity they end up caring for a weak, elderly stockman in the outback as they think he is their uncle. Their actual uncle is a neighbouring wealthy grazier who watches over them from a distance but wants to get a sense of their characters and ability to cope with adversity. He has a grandson, Gene, who befriends the teenagers.
I believe it is set in the late forties or early fifties. The copy I read was a slim green book without a dust jacket. It was quite an old looking book in the library of my small prairie high school in Manitoba in the mid-sixties.