The evidence was made public on Saturday as the decades-long search to find Jane, Arnna and Grant Beaumont, who vanished without a trace during an outing to an Adelaide beach on Australia Day in 1966, resumed.
Excavators took to the site of the former Castalloy foundry hours earlier, where it is believed owner Harry Phipps kidnapped, murdered and buried the children 59 years ago.
Independent MP Frank Pangallo organised the privately-funded dig and said a person close to the family has told him that Mr Phipps' niece married a cousin of the children's father, Jim, in the years after they vanished.
"That hasn't been previously known and it's another link in this case that shows Mr Phipps could have known the Beaumont kids and the family," he told Adelaide Advertiser.
"It's another piece of circumstantial evidence that points to (him) being the prime suspect."
Mr Pangallo was joined at the site in North Plymouth by forensic archaeologist Maciej Henneberg, author Stuart Mullins, former South Australian detective Bill Hayes.
Large earthmoving equipment also started preliminary excavation work at the northern end of the government-owned site, which is about to be sold off for development.
"This search will be the last opportunity to search the area where those closest to the case believe their bodies were buried before new industrial or residential developments are built on the site," Mr Pangallo said.
The search will revisit two sites previously excavated along with a third never-before-searched location and is expected to take about a week to complete.
Jane, 9, Arnna, 7, and Grant, 4, vanished without a trace from Glenelg beach on January 26, 1966.
Their parents, Jim and Nancy Beaumont, told police the children left home with sixpence but a shopkeeper recalled Jane buying pasties and a meat pie with a one-pound note.
Mr Phipps has long been the main person of interest in the children's disappearance and presumed murder.
Shortly after his death in 2004, Mr Phipps' son, Haydn, was interviewed and revealed he had seen three children at the family house shortly after the disappearance.
He also told police he was violently abused by his father as a child.
The new search will be the third time the Castalloy site has been examined.
In 2013, police excavated an area after claims by two brothers that three days after the children disappeared, Mr Phipps paid them to dig a "grave-sized" hole on the factory site.Â
At the request of Mrs Beaumont, police conducted a second dig in 2018, excavating to a depth of about 1.5 metres before abandoning the effort.
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