Hi there,
Robert C REED, aged 70 at Mosman in 1927, NSW BDM d.c. registration 19864, giving mother's name as Agnes T.
This would give birth year of around 1857, suggesting to me that he arrived on different voyage to Isabella M and her siblings and mum.
That d.c. should show how long he had been in the colonies, and may help determine when he and/or his father came to NSW.
It is likely that the R C Reed from the Sands directory of 1885 would be same Robert C Reed at Mosman in 1927.
Perhaps Agnes was a widow when she came to NSW in 1878.
Cheers, JM