“It almost looked like she was walking on clouds that day,” says Maren Sheldon, twin sister of the late breast cancer campaigner, Kris Hallenga.

Maren is remembering the “living FUNeral” her sister had organised last year in Truro Cathedral in Cornwall.

For the occasion, Kris wore a sequinned top with “YODO” on the back – You Only Die Once – and earrings in the shape of the poo emoji – a nod to her published handbook for living entitled “Glittering a Turd”.

Guests were dressed in sparkly sequins, a 20-piece orchestra raised the roof and Dawn French – as the Vicar of Dibley – gave a sermon at the lectern.

You could write your beautiful thoughts about Kris on a cardboard coffin and help yourself to a toy cat to cuddle – just as Kris relied on her own cat, Lady Marmalade, for comfort.

After the speeches, there was a disco, partying and tequila shots.

“It was amazing, it was the best day of her life, as she said,” recalls Maren.

“I think a lot of people say their weddings are the best days of their life, or the day that their children are born. She didn’t have either of those occasions in her life, so I guess her ‘FUNeral’ was for her that day.”

“She was so happy and so well at that time, too. I’m so glad she chose to do it then, before she got unwell.”

Scenes from the occasion feature in the new BBC2 documentary: Living Every Second: The Kris Hallenga Story – which is broadcast on 1 October.

Kris died in May this year at the age of 38. She had been diagnosed with terminal breast cancer 15 years ago and went on to found the breast cancer awareness charity CoppaFeel with Maren.

The charity encouraged young people to check their breasts and campaigned to get cancer awareness onto the school curriculum.

At the age of 22, Kris had gone to the GP with what she described as a “lumpy boob”, but was told it was nothing to worry about and likely to be hormonal. When she was finally diagnosed, a year later, the cancer had already spread to her spine. She also later developed brain tumours.

Her late diagnosis made Kris determined to prevent the same thing happening to anyone else. CoppaFeel was her greatest achievement and her message continues to save lives daily, Maren says.

In 2013, Kris made a film for BBC Three and, over the following decade, continued to allow the cameras into her life.

The new BBC Two film tells her story – from her early years to her final days – surrounded by Maren, Lady Marmalade and her friends.

“She wouldn’t want us to look back and think how tragic it all is, because that’s not how it was,” says Maren.

Kris was keen for death to be a conversation alongside living, says her sister, but the film – a celebration of Kris’s life – shows her “in all of her colours”, she says, and with all of the things that meant so much to her.

“It was about the simple things as well and it’s the contentment in her life that she was so keen for people to understand,” Maren explains.

“There was nothing left unsaid and nothing left undone, which I think is a pretty great place to reach when you’re at the end of your life.”

Kris wanted people to focus on everything she had achieved in those 38 years, says Maren. She changed the language we use around cancer and made it more accessible and not scary, she adds.

“She was this beacon of hope for so many people, especially people with secondary diagnoses,” says Maren.

“I guess she was an anomaly in that she lived for 15 years. But it’s not impossible. I think even if you don’t get 15 years, her point was that there’s life in the years that you have and to not waste a moment on things that don’t spark joy.”

While breast cancer awareness has improved and society has become more open about talking about our bodies, says Maren, the problem has not gone away.

“The fact is that it’s a treatable disease, but has to be caught at a treatable stage.”

Maren is now navigating a new life without Kris, which she says has made her realise how much she depended on her sister, and how much she needed her.

She is currently in the process of organising an exhibition in East London about Kris and the story of CoppaFeel.

Kris left it up to her to decide what would happen to her body after she died, says Maren.

“I think partly the reason I decided to go with cremation, is, that way, I could spread her [ashes] in all the places that meant quite a lot to her and to us and to other people,” she adds.

One day, after hosting a dinner with some friends to honour Kris and the brave conversations she had, Maren spontaneously felt the time was right to scatter some of them.

“We were going to get in the sea, because that’s what we always do – we live in Cornwall and we just love getting in the sea – and that was already part of the plan.”

She asked her friends how they felt “about taking a little shot glass of Kris into the sea” with them, which everyone wanted to do.

Kris always felt at home in the sea, says Maren.

“It just felt really nice to have her there with us as we did that.”

Life is like the sea, in many ways, sometimes it can be rough, other times, peaceful and calm. But it is Kris who summed up the beauty of living every second of it in a speech she gave to the congregation at her “FUNeral”.

“You see the thing about death is,” said Kris, it’s “so terribly final, whereas life – life is full of opportunities. So let’s seize these opportunities, let’s live fully, love deeply and make today count”.

Living Every Second: The Kris Hallenga Story will be broadcast on Tuesday 1 October at 21:00 BST on BBC Two and BBC iPlayer.

If you are affected by any of the issues raised in this story, support and advice is available via the BBC Action Line.