A glimpse at the “private, hidden face” of Albert Einstein, including the celebrated scientist’s thoughts on everything from his fears that his best work was behind him to his equivocal feelings about his fame, has been revealed in a cache of letters he wrote to his beloved younger sister, Maja.

The collection, which includes a previously unknown photograph of Einstein as a five-year-old and the only surviving letter written by Einstein to his father, comes from the archive of Maja Winteler-Einstein and her husband Paul Winteler. A mix of letters, postcards and photographs, many of which have not previously been published, the documents range in date from 1897 to 1951.

“What’s remarkable about them stems from the fact that he had this incredibly close relationship with his sister. It’s quite clear when he’s writing to her, there’s no role-playing at all,” said Thomas Venning at Christie’s, which will auction the letters at the start of May. “He was very conscious of what was expected of him after he became famous, and you don’t get any of that in letters to his sister. He says some things that I’ve never seen him say anywhere else, and I’ve catalogued many hundreds of his letters.”

In 1924, nine years after he completed the general theory of relativity in 1915, Einstein would write to Maja that “scientifically I haven’t achieved much recently – the brain gradually goes off with age, although that’s not so unpleasant. It also means that you’re not so answerable for your later years.” Ten years later, he would write to her: “I am happy in my work, even if in this and in other matters I am starting to feel that the brilliance of younger years is past.”

Venning said he had not seen Einstein admit this anywhere else. “It’s not him playing a role, you can see that thought going through his head. Which is true – if Einstein had died in 1916, his fundamental legacy would have been intact. He carried on working for another 40 years without making any other great breakthroughs. So it’s just an extraordinary moment which we get because of how close their relationship was. He didn’t have to reassure her,” he said.

Tackling topics from his hobbies of sailing and playing the violin, to his difficult relationship with his first wife, the letters are “unpublished snapshots of Einstein, his private face”, according to Venning. In one from 1935, Einstein makes a rare acknowledgement of his achievements, writing to Maja: “In our main avenues of research in physics we are in a situation of groping in the dark, where each is completely sceptical about what another is pursuing with the highest hopes. One is in a constant state of tension until the end. At least I have the comfort that my main achievements have become part of the foundations of our science.”

“It sounds unusually big-headed for Einstein – he was an incredibly low-key, humble person, always careful not to say anything that sounded too proud. But I think he felt he could say something to Maja,” said Venning.

In 1923, in a letter that Christie’s has valued between £6,000 and £9,000, Einstein writes to Maja of his international fame, telling his sister and her husband that “I am becoming very much loved and even more envied; there’s nothing to be done about it.”

“He’s not rejoicing in it, he’s just sort of accepting it. Einstein was the first scientist to be a world celebrity. Before that it just didn’t really happen to scientists, so he was in this unique position,” said Venning.

The shadow cast by the rise of the Nazis in the 1930s, and the strength Einstein drew from his work, is starkly depicted in a letter written to his sister in September 1933. Earlier that year, Einstein had renounced his German citizenship in Antwerp, fearing for his life after the Nazis branded relativity “Jewish science” and publicly denounced him. He took up a role at Princeton University in New Jersey in October; his sister would follow him in 1939.

“What will happen if we come back from Princeton next year? Will we even be able to? What will life be like there? The only unshakeable things are the stars and mathematics,” he wrote.

“This is him facing up to the fact his whole life has changed. He’s going to a country he doesn’t really know. And so his whole world is falling to pieces, and he says this wonderful line,” said Venning.

Christie’s will put the letters on view to the public from 18 to 20 April, and auction the collection online from 2 to 9 May.