@Paul B. I've been looking at this today. It appears like it's been bungled from day dot. Unfortunately Herald Sun articles are paywalled but I've tried to piece it together elsewhere.
A podcast from January 2019 featuring Andrew Rule, investigative journalist who has followed this case for years, is really informative.
The Unsolved 1984 Tapp Murders with Andrew Rule | Australian True Crime Podcast
Andrew Rule was recently contacted by the son of a paedophile who lived only 8 or 9 minutes from Margaret. He played night tennis on Tuesday nights (night of the murder) and also wore volley dunlops. The son has only just possibly linked his father with this crime. We don't have a name for this POI but Andrew is looking very closely at him now.
Another POI was a now deceased ex policeman who had personal links with Margaret, somehow he was able to enter the crime scene after the event and retrieve a book and or letters he had given Margaret.
A further POI is the partner/friend of Kaye B-S. He was a paedophile and the story goes that Mrs B-S hired him to kill Margaret. This came about because Margaret was having an affair with a Dr who owned the house Margaret lived in. When the Dr died Margaret went to court to retain the house but was awarded 1/2 along with the late Dr's wife and eventually buying the Dr's wife out. Wife then had bad blood about this and also allegedly found a sex tape with her late hubby and Margaret.
Oh, How Peculiar!
MURDER SEX TAPE LINK – Wednesday March 20 2013
Then to complicate things, there was dna found on Seana's gown but along the way it appears to be compromised with a man wrongfully being charged of their murder and then the charges withdrawn.
Newspaper NOW-Exposed: Unsolved Tapp Murders mother and daughter 1984 Ferntree Gully
Andrew Rule tracked Justin, Margaret's son, to England but when he got there Justin was deceased. Andrew did speak with Justin's estranged partner and children. It appears Justin could never cope with the murder of his mother an sister and felt that if he had been at home at the time of the murders somehow he could have prevented them.
How terribly sad. I know there were a lot of POI but it looks highly likely that one of the above POI's is the murderer. MOO