Hi Janet
I'm not sure what you've found since you posted in this thread, so you might already know what I'm posting. I know a bit about the family through the connection of Margaret Young, George's step sister.
The Melbourne newspaper Argus of 22 September 1855 lists the marriage of Margaret Young, "only daughter of the late Alexander Young, of Bristo-street, Edinburgh" to Rev. James Nish. In a notice for the same marriage in the Bendigo Advertiser Margaret's father is stated to be a baker. This is consistent with her father being the baker Alexander Young of 55 Bristo St Edinburgh who was the first husband of the Janet at the same address. (And this Janet also being the mother of George Summers.)
An obituary of Margaret Young (Margaret Nish after her marriage), has some details of the family history (Bendigo Advertiser 11 January 1915 "Death of Mrs Nish"). In it says: Margaret "was born in Edinburgh on 19th December, 1831, and arrived in Melbourne with her mother (Mrs. Young) and her four brothers on 8th November 1854. They came to Bendigo shortly afterwards, and her eldest brother (Mr. John Young) became well known as the proprietor of a crushing machine at Kangaroo Flat."
These four brothers seem likely to to be the brothers listed in the 1941 census record for "55 Bisto St" Edinburgh: George Summers, John Young, Alexander Young and James Young.
There is no mention of David Summers coming out to Australia in the obituary.
Margaret had a daughter Janet Henderson Nish who married in 1883. I presume the names 'Janet' and 'Henderson' were chosen because of Margaret's mother's given name and maiden name.
I have some family photographs taken in Scotland that probably belonged to Margaret Young or her mother but they are not labelled, so its hard to work out who they are.
Matt