The former fiancée of Gloria Vanderbilt's estranged son has revealed she will not watch the new HBO documentary about the famous heiress and her son Anderson Cooper.

April Sandmeyer said she was in tears all weekend after watching just the trailer for Nothing Left Unsaid, which premiered on the channel on April 9.

Sandmeyer was engaged to Chris Stokowski, Cooper's half-brother, who cut himself off from his family nearly 40 years ago after accusing his mother's therapist of meddling in his love life.

'I watched the promo. It was so painful,' Sandmeyer told Page Six. 'I had a good cry over the weekend and couldn't bring myself to watch.'

'It's very complicated and painful to rehash all the memories. It made me think a lot abut the past and everything that happened with Christopher.'

At the April 6 premier of the documentary, Nothing Left Unsaid, guests questioned how the film glossed over her son and Cooper's half-brother, Chris Stokowski. Above Vanderbilt and Cooper are pictured during the premiere

Christopher’s former fiance April Sandmeyer (above) told MailOnline that Chris spoiled Anderson rotten. She said: ‘Anderson was only 10 when his daddy died and his adored older half-brother disappeared. It’s heart breaking’

There's no real mention of Chris Stokowski (pictured right in college) in the HBO doc about Vanderbilt and Cooper, as her eldest son, Stan, is in the movie. Above left Vanderbilt and Stan are pictured in 2012

There is no real mention of Chris in the film, a slight that did not go unnoticed at the premiere, according to the New York Post.

Vanderbilt's eldest son Stan is featured in the film along with Cooper. Chris and Stan's 's father was famed conductor Leopold Stokowski, whom Vanderbilt was married to before Wyatt Cooper.

Anderson was very close with his half-brother Chris, who was 15 years his senior.

‘He adored Chris,’ Sandmeyer told MailOnline in an exclusive interview.

‘Anderson was only 10 when his daddy died and his adored older half-brother disappeared. It’s heart breaking’.

MailOnline revealed the true heartache behind the bitter dispute that broke up his family.

Railroad heiress Gloria doted on Anderson and his older brother Carter, the sons from her fourth marriage to writer Wyatt Cooper, and has often referred to them as her 'golden' boys.

She is also close to Stan, a 63-year-old landscape gardener, and his three children. Yet nowadays she does not acknowledge his brother Christopher.

In her 1996 memoir A Mother’s Story there is no mention of him.

The book tells of her heartache over Carter, who committed suicide aged 23 by leaping out of her 14th floor Manhattan apartment. She dedicated the book to Anderson.

And in an earlier autobiography, It Seemed Important At The Time, she details her four marriages and flings with Frank Sinatra and Marlon Brando but only mentions her two eldest sons once.

Gloria married Stokowski, the musical genius behind Walt Disney’s Fantasia in 1945. In her memoirs she refers to the world famous orchestra leader as God.

But their marriage was a tumultuous one and following her affair with Sinatra they divorced ten years later. Gloria also fought for custody of Stan and Chris, then aged five and three.

An aspiring actress and artist, Gloria finally found the happiness and success she’d craved her entire life when she met magazine editor Wyatt Cooper, who came from a family with no money.

They married on Christmas Eve 1963 and Gloria was soon pregnant with Carter and then Anderson. By the mid-1970s she was also a household name with her signature fashion jeans.

Chris, who was painfully shy as a child and led a solitary life at Bard College, hated his mother’s newfound fame. He also shied away from using his surname as he tried to carve out his own career as a musician.

While his gregarious older brother Stan moved in with his girlfriend and played drums in a jazz group, Christopher lived at home with his mother’s new family and played in a band at New York’s fabled Max’s Kansas City under a fake name.

In 1974, he fell in love with socialite Sandmeyer, and the two were planning marriage when Chris’s father, then in his 90s and living in the English village of Nether Wallop, fell ill.

The duo moved to Europe, splitting their time between Britain and Stokowski’s estate in the South of France.

Stokowski, who once had an affair with Greta Garbo, was 95 when he died in September 1977.

Chris inherited a large chunk of his father’s vast recording fortune but back home in New York his life quickly unraveled.

Chris and Stan's father was famed conductor Leopold Stokowski (above), whom Vanderbilt was married to before Wyatt Cooper. Gloria married Stokowski, the musical genius behind Walt Disney’s Fantasia in 1945

Their marriage was a tumultuous one and following her affair with Sinatra they divorced ten years later. Gloria also fought for custody of Stan and Chris, then aged five and three. Above she is pictured with Sinatra

His stepfather Cooper died months later, aged just 50, and Gloria fell under the spell of a handsome therapist called Dr Christ Zois.

In her memoirs, Gloria describes him as a young Ryan O’Neil lookalike and tells how she paid for him, his family and lawyer friend Thomas Andrews to fly to France on Concorde.

She lavished them with grand holidays and Cartier watches. They were frequent guests at her homes in Manhattan and the Hamptons – and Zois constantly offered advice on her children and their relationships.

April was so shocked and upset when she discovered Zois had been meddling in her relationship and making comments about her that she broke up with Christopher.

She still finds it hard to describe what happened. 'Christopher was the love of my life,' she told MailOnline.

‘But I was so devastated when I discovered what Zois had done that I felt I had no choice but to break things off with Christopher. I can’t tell you what Zois said, it is just too personal. But I was heartbroken’.

Chris, then 26, finally broke away from his mother and moved into an apartment on East 44th Street to try and win April back.

An aspiring actress and artist, Gloria finally found the happiness and success she’d craved her entire life when she met magazine editor Wyatt Cooper, who came from a family with no money. Above Gloria and husband Wyatt Cooper and sons Anderson (left) and Carter are pictured

When she refused to see him, he informed his mother that he was leaving New York for good.

'He just wanted to get away. It was all too much for him,' said a former associate.

'Chris never felt like he was loved and April felt the same – they were two peas in a pod. Then Gloria’s shrink got in the middle and messed everything up.

'Chris just took off – at first he went to his father’s old farm in Nether Wallop, then he headed for the hills. He was in Massachusetts or New Hampshire, then Vermont. But he did not tell his family any of that.'

Ten years later when April attended Carter’s funeral in 1988 Gloria rushed up to her and asked her where Chris was.

'She thought we had gone off and been together the whole time,' says April. 'She had no idea we had broken up.'

'It was very strange.'

Sandmeyer told Page Six there was a 'lot of sadness in that family' and that much of it came back when she saw the documentary's trailer.

'A lot of the sadness had to do with thinking about how Dr. Zois impacted our relationship and my future and Christopher’s future, our lives,' she said.

'One thing I want to do before I die is apologize to Christopher for ignoring his phone calls and letters,' she said.

'I've carried this guilt around for 38 years.'

Sandmeyer revealed she had been in touch with Chris through a mutual friend, but would not elaborate.

Gloria eventually discovered that Dr Zois and the lawyer Edwards had ripped her off to the tune of millions. She successfully sued them but never received a penny of the $1.6 million they were ordered to pay back.

Edwards died of cancer shortly afterwards but Zois was stripped of his medical license after he was convicted of bank fraud and conspiracy to commit bank fraud.

Wyatt Cooper died suddenly in 1977 and Carter Cooper (far left) committed suicide in 1988

Gloria discovered that Zois and Edwards had not only sold off her $10 million-a-year fashion and home furnishing business behind her back, they had also failed to pay any of her taxes.

She was forced to sell her homes to repay the taxman but battled back to forge a second career as a memoir writer and artist.

Chris was also a talented artist, creating shadow boxes and other small pieces. But he was too shy to exhibit under his own name.

‘Wherever he is, he won’t be using the names Stokowski or Vanderbilt’, said his associate from the 1970s.

‘Even in the center of Manhattan he was hermit-like. He was a regular at Max’s Kansas City – the hottest club going – but no one knew his name.

‘The Rolling Stones, David Bowie and Andy Warhol were regulars. Chris sometimes played on stage but he used a fake name.

‘His father was world famous, a legend in classical music circles, and he didn’t want to be judged on that.

‘And his mother’s name was embroidered on the back pocket of every other pair of jeans. He used to walk down the street counting the number of women wearing her jeans.

‘Then his father died, his mother got involved with the shrink and April broke up with him. It was all too much. He had to get away.

‘I feel sorry for Anderson. He adored Chris, who spoiled him rotten buying him LEGO sets and other fancy toys. They spent every summer building sandcastles on the beach.

Gloria has only spoken once publicly about Chris, telling the Daily Telegraph in 2004: 'He cut himself off completely from all of us. He told us what he wanted to do and he’s done it.

‘When Carter died I thought he would come back but he didn’t. And we respect his wishes.’

Stan, a renowned landscaper in the Hamptons whose work has been featured in Architectural Digest, told the MailOnline: 'I really don’t want to talk about this.'

Anderson’s spokeswoman Lauren Varney said: ‘We do not comment on our client’s personal life’.

Gloria did not respond to a message left at her apartment building. Zois, now 74 and a scriptwriter, did not return calls.

Chris’s last known address was a post office box in Montpelier, Vermont.