Previously unpublished letters written by Albert Einstein are to be auctioned off by Christie's in May – including an unseen photograph of the famous scientist as a child.

The letters, which date from 1897 to 1951, were correspondence between the theoretical physicist and his younger sister, Maja Winteler-Einstein.

Thomas Venning, the head of the books and manuscripts department at the British auction house, told the Guardian that the Jewish scientist “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.”

Einstein completed his general theory of relativity in 1915. Nine years later, at the age of 45, he wrote to his sister, “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 year”.

Other letters contain a discussion of the situation in Europe with the rise of the Nazis in the 1930s. In 1933, Albert Einstein renounced his German citizenship after the Nazis publicly denounced him and his “Jewish science”. He moved from Germany to the United States later that year, joining the Institute of Advanced Study at Princeton.

“What will happen if we come back from Princeton next year,” he wrote to his sister.

“Will we even be able to? What will life be like there? The only unshakeable things are the stars and mathematics.”

The letters will be available to view by the public from April 18-20, with the collection being auctioned online between May 2 and May 9.

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.