This Ellen clearly wasn't a sister, but doesn't appear to have been a niece either (I have all theri names).
All the names of the nieces of whom? If Ellen was the dau of one of Peter's brothers then she would be a niece.
Contact the Adelaide Cemeteries Authority re their registers and burial orders for Ellen/Helen. Perhaps there was a parish burial record as well ie if a Catholic priest officiated at her burial. Her correct age at death is necessary.
The signatures of the witnesses on Eva's marriage certificate may be helpful also. Her father was described as Edward
HYDE of North Adelaide....if he existed surely he would have given his consent for his dau who was a minor, to marry. I wonder why a 19 year old ?Catholic girl married at the Wesleyan Parsonage in North Adelaide and not a church.
You are correct that South Australian Certificates have scant information and there is little more than is available on indexes which many of us have on CD. The Colony of South Australia adopted the English system of recording bdm information. There is also a BDM index on SAGenealogy and for a fee they will send you all the information on the certificates.
Information on South Australian Certificates
http://www.rootschat.com/forum/index.php?topic=368744.0Including info on death certs...
1908-1947
Date and Place of Death
Name
Sex
Age
Occupation
Usual Residence
Birthplace
Length of Residence in Commonwealth
Age at Marriage
Number of Issue Living and Deceased
Cause of Death
Place of Death
Informant
Wonder at what age she married?
I didn't post the death registration as I assumed you would have at least purchased this certificate.
HYDE Ellen 65 years Status Widow Relative Edward HYDE [Deceased Husband]
26 May 1917
Residence New Parkside Death Place New Parkside Ade 410/288
It is possible that none of the story is true. People often claim to be related to well known people because they shared the same surname. Family stories
Wonder what is the place of birth and length of residence in the Commonwealth on this death certificate?
PRITCHARD Eva 51 years Status Married Relative Frederick PRITCHARD [Husband]
9 Dec 1923
Residence Parkside Death Place North Adelaide Ade H 467/130
Symbol H = Died in hospital or place of care.
Transcriptions are available from SAGenealogy or Graham Jaunay. Google for websites.
Cando