The paper is to study the correspondence by A. D. Kantemir with his sister, M. D. Kantemir, in the context of development of the epistolary genre in Russian literature of the 18th century. It turns out that it began to advance to leading positions as early as the first third of the 18th century (as evidenced, in particular, by the writer of the time of Peter the Great “Annexes, how to write different compliments...”), a friendly letter plays a decisive role in the communication between the prince and the princess. This is a kind of family-friendly correspondence. Its character changes over time: in 1732–1740 it is mostly confidential and confessional, in 1741–1744 — business. Probably, such changes occurred under the influence of external factors: the state of health of Kantemir was steadily deteriorating, what he tried to conceal from his sister not to upset or frighten her, yet he did not want to lie to her either. At the same time, he had neither the strength nor the time to discuss anything, except the cases when he was in a hurry to arrange, feeling the approach of death. Obviously, for these reasons, personal topics were hushed up. But even transferred to the business register, the correspondence between brother and sister Kantemir remained invariably friendly. Whatever character it wore, it displays such features as empathy, sincere participation of one correspondent in the events of the life and experiences of another (hence the special warmth in appeals to the addressee, including in prescripts and clauses), the presence of friendly jokes, as well as its own kind of “secret writing”, an allegorical language, understandable only to the correspondents. Princess Maria suggested that Cantemir adopted nicknames for mutual acquaintances, and in the letters of Princess Maria these nicknames realize their literary potential. They are two-pronged (the Princess alternately actualizes the “human”, then the “animal” plan of content, long anticipating the method of switching the narrative from the “animal” to the “human” plane, characteristic for the satire of M. E. Saltykov-Shchedrin, including his tales) and are quite satirical. The presence of literary — first of all, poetic — insertions is considered to be one of the features of friendly literary writing, the appearance of which in Russian literature is traditionally dated to the second half of the 18th century. The correspondence between the Prince and Princess Kantemir is closely connected with the tradition of sentimentalism, thanks to its confidential, confessional and friendly tonality. The study demonstrates that this correspondence enriches academic understanding of the essence and history of formation of the friendly literary writing.