It is understood the siblings had travelled to their motherland for a family gathering in their village of Saleimoa. They had been sightseeing through Lalomanu - severely damaged in the 2009 tsunami - when they crashed.
Police removed the vehicle this afternoon.
A witness who did not want to be named described the van as being "completely totalled."
"We just saw the van being taken by the tow truck. It was really smashed - the whole front. The steering wheel was out the front window. It's completely smashed.
"The back of the van was fine...but the front was just totalled."
Assistant Police Commissioner Fauono Tapu told the Herald officers were still investigating the cause of the crash, but acknowledged that it was a horrific accident.
"There are three brothers and a sister, who is from New Zealand. This is probably one of the worst crashes we've had in Samoa for a long time," Mr Tapu said.
"Not only that, it's one family - it is very sad."
A Ministry of Foreign Affairs spokesman confirmed Mrs Faofua, a New Zealand citizen, died in the accident and the consul from the NZ High Commission in Apia was in touch with her next of kin.
Tributes have already started flooding in for the four siblings and their extended families in the US, Australia and New Zealand.
Ian Faleono wrote on Facebook: "My deepest condolences to the families who have lost loved ones due to the accident yesterday. Our thoughts and prayers are with you all [through] this time of sadness."
Emma McCarthy wrote: "R.I.P Paulo, Atonio, Manuele and Valelia."
