The family of two siblings, whose deaths shook the community of Llangolman in the 70s, is supporting a campaign for Dyfed-Powys Police to reopen enquiries into their deaths, saying they are 'convinced' that they too were victims of Pembrokeshire serial killer, John Cooper.

Retired farmer Griff Thomas and his sister, Martha, were found dead in their burning farmhouse in in Ffynnon Samson in 1976.

A coroner returned verdicts of manslaughter on Mrs Thomas, aged 70, and an open verdict for her 73-year-old brother.

"At the time it was deemed Griff Thomas had murdered his younger sister, Patti, and then set fire to himself and that there was no third person involved," said editor of Welsh language newspaper, Clebran, Hefin Wyn.

Clebran, along with Mynachlog-ddu Community Council, has asked Dyfed-Powys Police to review the coroner's verdict and reopen of the case, given the advances in forensic science.

However, at last month's Policing Board meeting, police and crime commissioner Dafydd Llywelyn was told by the temporary chief constable that the force would not re-examine the deaths without concrete new evidence.

"They will only examine any specific new information containing detailed knowledge or evidence, and any further decisions would be based on the results of the examination of that new material," he said.

"The temporary chief constable also noted that there is no intention to re-investigate any incidents on speculation alone."

Now Griff and Patti's second cousins are joining the call for their deaths to be reinvestigated.

"There is clearly an injustice here," said second cousin Huw Gibby.

"Griff and Patti were deeply religious. It seems extremely implausible that anyone would kill their sister then set fire to himself.

"The villagers who knew them also feel that it is totally implausible. The police decided that it was not a robbery as no money obviously taken so the offender was not Cooper - this seems a very flimsy basis to eliminate Cooper."

Mr Gibby pointed out the similarities between the Ffynon Samson case and that of the Scoveston Manor murder, that Cooper was convicted for in 2011 following a cold case review.

"Both were remote farm houses; both elderly vulnerable victims; both were believed to have had visits to the nearby areas by Cooper," he said.

"My two cousins and myself, following the successful cold-case review of the Pembrokeshire Murders case, are convinced that Cooper was the offender in this case."

He thanked Clebran for raising the matter and called for a similar cold case review to be undertaken for Ffynnon Samson.

"If there are any items retained from this case, the possibility of DNA evidence should be examined," he said.

"Even if there are no retained items, the case should be thoroughly reviewed as a cold case, not dismissed as pure speculation. "

Mr Gibby has also written to the police and crime commissioner urging him to push for the case to be reopened.