Trying to draw implications from the letter.
When Martha wrote the note in 1907 she was 23 in Sydney.
LILY is living with or near GRANNY.
Which Granny?
Granny Martha did not die until 1917. So it could be her in Victoria.
Granny Margaret? SHAKEL, when did Margaet Upton die?
"MUM" refers to the mother of both LILY and MARTHA (the note's author)
If MUM, in regard to the letter,has to "send it back" she is not living with or near Martha who has mailed it to her to read.
I wonder who is SA who sent the letter from Kimberley .
There is Samuel (born 1872)who would be the uncle of the girls.
Whoever SA was he/she was obviously of interest to the whole family.
Just thoughts

Sue