He became involved with Elsa, a cousin, in 1912 when he was still married to his first wife Mileva, a fellow scientist with whom he had two boys, Hans Albert and Eduard. Before they married, they also had a daughter, Lieserl, who was given up for adoption.
Einstein divorced Mileva and married Elsa in 1919, but within four years he was already involved with Bette Neumann, his secretary who was also the niece of one of his friends. Many more liasons followed.
The letters reveal how one of his women, a beautiful Berlin socialite called Ethel Michanowski, followed him to Oxford, only to discover that he was involved with a another woman.
Einstein discussed his extra-marital affairs openly in letters to his daughter and his wife. ...
Einstein's distance from his two sons after the divorce from Mileva clearly troubled him. He writes how much he enjoys taking the boys on holiday but at times expresses despair at his younger son Eduard, who suffered from schizophrenia. On more than one occasion he suggests it would have been better if Eduard had never been born. ...
The divorce settlement with Mileva contained a unique clause, in which Einstein agreed that should he win the Nobel Prize he would deposit the money in a Swiss bank account in Mileva's name and she could use the interest to finance the upbringing of the children. Einstein failed to fulfill this promise, and Mileva always felt betrayed.
The newly-released [in 2006] papers reveal that he invested three-quarters of the money, some $24,000, in long-term bonds via the Ladenburg and Thalmann Bank in New York. Mileva was supposed to receive the interest. But the value of the bonds were wiped out in the American Depression of the 1930s and Mileva's income dried up.
--Matthew Kalman, Daily Mail, on Einstein's moral failings