Australian authorities are providing assistance to the family of a South Australian woman who is suspected to have died while sailing off the east coast of Africa.

Deirdre “Cookie” Sibly, 67, from Port Lincoln, had departed from Reunion Island, a French territory in the Indian Ocean, in June. She was travelling with her friend, Pascal Mahe, a French national, on his yacht and the pair had intended to sail to South Africa.

Sue Good, Sibly’s older sister, said that on Wednesday evening local time a distress call was made from the yacht – named the Acteon – in the Mozambique channel, about halfway between Madagascar and Mozambique. The vessel was later found adrift with two deceased people on board.

Recounting events since the distress call, Good said that a container ship had picked up the call and alerted the French coastguard, which then contacted Sibly’s family in Australia, who tracked the container ship’s movements all of Thursday and Thursday night.

She said the cargo ship’s crew was not able to board the yacht, instead trying “everything” to contact those on board. But “there was no sign of life on the boat”, she said.

That ship was then joined by a cargo ship and a maxi yacht. Cook said she believed sailors on the maxi yacht were eventually able to board the Acteon.

“I had a call from [the Department of Foreign Affairs and Trade] at about 10am Friday morning, that’s when they told me someone was able to board the yacht and a man and woman had been found deceased,” she told Guardian Australia on Monday afternoon.

The pair was yet to be formally identified and a family member may have to travel to Africa to identify Sibly’s body, she said.

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As of Monday, Good said the yacht was “still drifting along in full sail with them on board”. She believed the vessel was being tracked by satellite.

Given the pair were both “very experienced” sailors, “for sure something very wrong happened”, she said.

It is understood officials in South Africa and Mauritius are working with local authorities and French officials on the investigation.

She said the investigation and repatriation was “happening so slowly”.

“We would like answers to lots of questions and the next thing is to get her back here,” she said, adding that she had been warned the process would be lengthy and could take “months”.

She said her sister had been sailing for 40 years and was “having the time of her life”, while her companion, whom she met earlier this year when he sailed to Port Lincoln, had sailed around the world for about a decade, much of that time solo.

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During the voyage, Sibly contacted her family every time she was on land and stayed in touch via the travel app, Polarsteps. The pair planned to sail to Durban, then on to Cape Town, from which she would fly home.

“She last sent me a message on 19 November, because they were about to leave port to keep going south. She said they would be sailing at least for 14 days,” Good said. “She knew it was dangerous, but she was really adventurous. She loved sailing, it was her whole world.

“I hope it’s all wrong,” she said of the turn of events, “but I reckon there’s about a 1% chance it’s wrong”.

A Department of Foreign Affairs and Trade spokesperson said it was “providing consular assistance to the family of an Australian missing in the Mozambique channel”.

“Our thoughts are with the family at this distressing time.

“Owing to our privacy obligations we are unable to provide further comment.”