A woman has waived her right to anonymity to describe how she and her sister were sexually abused as children by a monk on Caldey Island, calling for an inquiry into how offences were covered up for decades.

Joanna Biggs also claims that a nun lied about the circumstances surrounding the death of her sister, Theresa, at the age of six in a swimming accident on the island 40 years ago, and wants her inquest reviewed.

Biggs is one of a growing number of survivors who have come forward to detail offences committed by Fr Thaddeus Kotik, a member of the Cistercian order of Benedictine monks who lived at Caldey Abbey on the Pembrokeshire island from 1947 until his death in 1992.

Since the Guardian exposed the abuse in November there have been calls from victims and politicians for an inquiry. The Guardian has also revealed that two other men who lived and worked on Caldey were subsequently convicted of child sex offences.

In addition, police are investigating allegations that a seasonal worker sexually assaulted a girl on the island.

Biggs, 48, is the first Kotik victim to be named. She said one of her main motivations was to defend her sister against the accusation that she disobeyed the instructions of a nun and went swimming when she had been told not to go in the water. But Biggs said she and her sister were abused by Kotik, in the dairy on the island, when they were aged six and seven.

Kotik’s modus operandi was to befriend families who lived there or who regularly visited. He gave them handmade chocolates and fresh produce, and invited the children to the dairy. He abused children in a room beside the dairy, on walks through the woods, in dens or in isolated rocky coves near the beach.

Biggs said that although she was the older sister, it was Theresa who suggested they avoid Kotik.

“We were always together,” she said. “We played together and talked about everything together. Although I was older, Theresa was more gregarious and bolder. Theresa suggested we should stick together for protection.

“I have a memory of me being in a huddle with her in a garden somewhere by ourselves and her saying that we shouldn’t go to see Fr Thad, even if he gave us sweets, and me nodding in agreement. I knew why she was saying that at the time. I didn’t like what Fr Thad did.

“I feel grateful that she was the one who voiced it. This helped me avoid a different paedophile about a year or so later – not on Caldey, on holiday somewhere else – it flashed into my head like a warning and I listened and ran away from him.”

Biggs told how she and her sister had once begun to act out the abuse they suffered. “We started to re-enact what happened in the dairy to us, taking our pyjamas off – and then we stopped and decided we didn’t like it and didn’t want to play that story.”

Biggs said the Guardian’s original article triggered panic attacks. “It was the picture of Fr Thad holding the two girls – they’re not Theresa and I, but you can see how tightly he held them in his arms. He was very strong and had rough hands. The most bizarre place I had a panic attack was in a supermarket one weekend, next to the freezer section. I was suddenly overwhelmed by the cold, the smell and the noise.”

Thaddeus Kotik with two children on Caldey Island

In July 1977 the two sisters were among a group of children at Sandtop Bay beach on Caldey under the supervision of a nun called Sister Sheila Singleton, who was leading an educational course for Catholic children.

Theresa went swimming on a windy day unaware there was a dangerous undercurrent and she was swept out to sea. Three boys, 12, 14, and 15, swam out to try to help her but she drowned.

During the inquest at Tenby police station, Singleton, who died in 2004, testified in a written statement that Theresa had defied her instructions not to go swimming because the water was too cold. The boys told a different story but the coroner accepted the nun’s version and recorded a verdict of accidental death.

Biggs insists that Singleton’s evidence was false because the nun had given her sister permission to swim. “She [the nun] helped me to put Theresa’s armbands on. And then she said, ‘Off you go.’ My sister was not naughty.”

Biggs said she and her parents have lived with the nun’s lie for more than 40 years. “It is time for my sister to be released from false blame,” she said. Biggs argued that the story of her sister’s drowning was relevant to the abuse scandal because it showed that children’s voices were often ignored.

“The problem with things as they currently are is that everything to do with Caldey has been closed off, unmentioned, swept under the carpet or just whispered about for a very long time.”

The abuse on Caldey emerged after the abbey paid modest sums of compensation to six women who had brought a civil claim against the Cistercian order on Caldey for abuse they suffered as children by Kotik in the 70s and 80s.

After the Guardian exposed the abuse last month, the abbot, Daniel van Santvoort, apologised via the island’s Facebook page that allegations made against Kotik had not been reported to the authorities and expressed regret for any harm caused.

Since then the Guardian has revealed:

• A sex offender called Paul Ashton lived on the island for seven years while on the run from the police. It has also now emerged that he ran a cleaning company registered on the island with a convicted fraudster.

• A priest, Fr John Shannon, who was subsequently caught on the mainland with pictures of children as young as nine, lived on the island for nine months.

• Police are investigating another man over an alleged sexual assault that took place at the same period as the Kotik offences. He was not a member of the abbey or its staff.

Adding her voice to calls for an inquiry from other victims and Tory Welsh assembly members, Biggs said: “I feel like it’s just the start, the box is only just being opened. It seems to me that the best people to investigate that box fairly and thoroughly would be people and organisations who have not been previously associated with the island – and also who do not belong to a particular denomination or faith.

“I think this would be best for Caldey Abbey as well. If they want to try and win back people’s trust that they are truly interested in safeguarding their visitors in future.

“The fact that Fr Thad was left for so many years to continue to do whatever he liked suggests at the very least, tolerance or blindness by Caldey Abbey to that same behaviour in others. These are offenders who understand Catholicism and know how to hide within it and manipulate it.”

Biggs added: “There appears to be a firm position taken by the current abbot towards protecting the interests of the monastery, and a distinct lack of openness, clarity and even goodwill when it comes to the dealing with revelations of the abuse that took place. His order is responsible for covering it up and allowing it to continue. Acknowledgment is everything.”

It can also be revealed that the abbey’s wealthy “mother house”, Scourmont Abbey near Chimay in Belgium, denied legal liability for the abuse. Caldey’s webpage spells out how the island was sold to the Cistercian order in the 1920s to be occupied by a group of monks from Scourmont and adds that the present monks are the successors of the first Caldey Cistercians.

The Guardian has also established that Kotik spent time at Scourmont. But Armand Veilleux, the Scourmont abbot from 1999 until his retirement in November this year, said Caldey was an autonomous house in civil and canon law.

Teresa Elwes, a devotee who has maintained a relationship with Caldey and Scourmont for 40 years and knew Kotik, said the mother house – which is known for its brewery – could afford to compensate victims properly.

Elwes said: “These young women have been abused by a monk and now that abuse is continuing by the failure of the monastic community to take responsibility and willingly and eagerly pay proper compensation, whilst acknowledging that this alone can never be enough.”

The Caldey Abbey abbot has not responded to requests by the Guardian for an interview and has not granted permission for reporters to land on the private island.