He did much to shape late 20th-century history as a towering figurehead of German unification and one of the key architects of the euro. But former German chancellor Helmut Kohl has now become almost completely isolated from the outside world, including from his only grandchild, with his two sons accusing his much younger wife of "imprisoning him".

Walter and Peter Kohl have publicly accused Maike Kohl-Richter of controlling their father's life to the extent that she writes his letters, decides who is allowed to see him, blocks access to some of the former German leader's once closest confidantes – including his former chauffeur and photographer – and chooses to keep him "like a prisoner" in the bungalow they share.

The revelations of the family drama that are gripping Germany like the plot of a Thomas Mann novel, are laid out in the new 27-page foreword of a book about Hannelore, their mother, who took her own life in 2001 after suffering from a crippling light allergy.

The 2002 bestseller Hannelore Kohl: Her Life is being rereleased in time for what would have been her 80th birthday this week (7 March).

Speaking on a chat show on national television last Thursday night, the brothers said they wanted the story of what they called their father's "tragic downfall" to be told, as well as details of the "increasing loss of control he has over his own life". Describing how relations between the 82-year-old father and his sons had broken down, Peter Kohl expressed the hurt he felt on having to read about his father's wedding to Richter in May 2008 in the tabloid Bild, whose editor, Kai Diekmann is reportedly one of the few people who still have access to Kohl.

Peter, the younger sibling, fought back tears as he said he hadn't seen his father since May 2011, describing how with his granddaughter he had tried to visit him at his home in Oggersheim, near Ludwigshafen, in south-west Germany. "The door was opened by a very indignant Maike Richter," he said. "She signalled to me to go into the living room where my father sat in his wheelchair, clearly really happy to see my daughter again. He held her hand and then after around 10 minutes said: 'It's better that you go, otherwise I'm going to get into trouble again'."

In a recent article, Spiegel went so far as to describe 49-year old Kohl-Richter as the "Lady Macbeth of Oggersheim". "She is the gatekeeper, the person who controls his words … decides who gets in and what gets out," the scathing article stated. The sons described how their happiness for their father when he told them he had met a new partner, even one 34 years his junior, quickly turned to "queasiness" when they realised the "unhealthy degree" to which she apparently idolised him.

Meeting her in Berlin for the first time in 2005 at the behest of their father, Peter described the brief stop-off they made to Richter's flat on their way to a restaurant. "For a few minutes I had the opportunity to see the inside of the flat of this woman who was almost the same age as me. In my male naivety I was really not prepared for what I saw," he writes in the book.

"I stumbled into a sort of private Helmut Kohl museum. Wherever one looked Helmut Kohl photographs were either hanging or standing. There were pictures of the two of them, pictures with him and others … election memorabilia, a letter with his signature preserved behind glass, and other Helmut Kohl artefacts." He added that the whole scene resembled "a decades-long collecting passion for the purpose of hero worship like you might attribute to a stalker".

The brothers have also disclosed how a trusted aide to Helmut Kohl revealed to them at his 75th birthday celebrations that the affair between their father and Richter, who first met when she worked in the chancellery, had been going on since the second half of the 1990s, while Hannelore Kohl, to whom he was married for 41 years, was still alive.

Neither has the woman, who is said to have held a candle for Kohl ever since joining her local branch of the youth wing of his Christian Democrats (CDU) as a teenager, done much to endear herself to the German public, not least with her decision to wear the clothes of the former chancellor's wife, who enjoyed huge popularity.

Peter Kohl described the many well-documented clothing incidents as "creepy", adding, "but it's even worse than that. She's taken to wearing certain pieces of my mother's jewellery that go back five generations in the family."

The siblings trace their father's increasing inability to control his own life down to a near-fatal fall he suffered at his home in 2008, which left him virtually paralysed and with impaired speech.

But some say it is that very incident that proves just how much the elder statesman needs Kohl-Richter by his side. Dirk Metz, a leader CDU politician and an old friend of Kohl-Richter's from her hometown who originally invited her to join the Young CDU, is one of the few still allowed to visit Kohl at home. In an interview with the Süddeutsche, Zeitung he recalled a moving exchange with the ageing politician in his garden, in which Kohl reportedly said to him: "Without her I wouldn't be here anymore".

Kohl-Richter, an economist who has been on leave of absence from her job at the economics ministry so that she can care for her husband, has so far not responded to the sons' accounts or to repeated requests by German journalists to interview her, which have been met with polite written refusals from Helmut Kohl's Berlin office. But some suspect she might soon be forced to break her silence.

It is speculated she may have been behind a statement supposedly issued by Helmut Kohl in 2011, in which he called portrayals of his private life at a time when rumours abounded that he was estranged from his sons, "inappropriate and inaccurate". He said: "I feel the marketing of and public displays about my private life by third parties are inappropriate."

Political historians have expressed their concerns that Kohl-Richter has almost complete control of Helmut Kohl's archives, with most of his letters – everything from the correspondence between him and Hannelore, letters to and from him and the likes of George Bush Sr. and François Mitterrand or the handwritten condolence letter from Prince Charles after his wife took her life – stored at Oggersheim.

Those close to Kohl, including his would-be biographer Heribert Schwan, say they fear she could choose to edit his life and therefore shape his legacy, as she sees fit.

The brothers, who say the proceeds from the book will go to the foundation their mother set up in 1983, but with which their father has officially ceased all contact, say they have gone public with their concerns for the sake of her memory.

"In her suicide note her message to us all was 'tolerate each other', but we have failed to do that," said Peter.

"I think of the decades in which my mother strained to keep the family together. I could never have thought such a closely-knit family was capable of falling apart."