We know almost nothing of Susan.
From one of Mac Bridges letters we can identify Susan as "Lammie" - so called because she liked to care for the lambs as a small girl.
Rebecca Spencer (nee Hooper) mentions Lammie in her 1879 letter to Eliza Saunders saying that she makes cheese.
May Spencer has many references to Aunt Lammie in her 1887 diary. She is living in Muswellbrook (persumably with her parents) and helps out in various relatives houses. Only once does May refer to her as Aunt Susie. She comments that she does not think that her Aunts Sarah and Susie will ever change their name (marry).
There is a reference to her by name in "Aunt Mary's Story" by Edna Mary Spencer sying that she used to go to Cuttabunda when Matthew was there. An anecdote also describes how Susan's umbrella is appropriated by Matthews sons Aubrey and Granville to make arrows to shoot at a neighbours bull.
Susan did not marry.