From The Boston Globe:
According to excerpts of letters made available to reporters, Einstein discussed his extra-marital affairs openly with his family. “It is true that M. followed me and her chasing after me is getting out of control,” wrote Einstein to his stepdaughter in May 1931 of Michanowski’s infatuation. “I will tell her that she should vanish immediately. . . . Out of all the dames, I am in fact attached only to Mrs L. who is absolutely harmless and decent, and even with this there is no danger to the divine world order.”
“I don’t care what people are saying about me, but for mother and Mrs M. it is better that not every Tom, Dick and Harry gossip about it,” he wrote. “Mrs L.” was Margarete Lenbach, another wealthy woman who used to send a chauffeur-driven car to collect Einstein for their late-night trysts. But Einstein valued Michanowski’s discretion, as he wrote to his second wife Elsa in 1931.
“Mrs. M. definitely acted according to the best Christian-Jewish ethics: 1) one should do what one enjoys and what won’t harm anyone else; and 2) one should refrain from doing things one does not take delight in and which annoy another person. Because of 1) she came with me, and because of 2) she didn’t tell you a word. Isn’t that irreproachable?”
More here.