Susan Loxton was born on Nov 2 1862 recorded at Mount Pleasant, South Australia, Australia. Susan's Mother was Ann Lintern, the youngest sister of Elizabeth Grace Loxton. Ann was 17 at the time. Susan was brought up in the Loxton household and always called herself a Loxton.
Susan has 2 entries in the SA Birth Records: One as Susan Lintern, the other as Susan Loxton. Both records show the Father as Albert Loxton.
SA Birth Records: District of Talunga, Book 28 on page 84.
There is nothing to indicate that this might have been a convenience rather than an admittance. The answer is lost in time.
There is no such person as William Joseph Loxton. Albert and Elizabeth's first child was Alfred John Loxton. Their next child was William Charles Loxton. Both of these children were born in the UK and came out with their parents.