Second cousin of the composer (b. 9/21 December 1868; d. 1912), born Count Aleksandr Nikolayevich Litke (Александр Николаевич Литке), and known familiarly to Tchaikovsky as Sanya (Саня).
Correspondence with Tchaikovsky
One letter from Tchaikovsky to Aleksandr Litke has survived, dating from 1891, and is available in English translation on this website:
17 letters from Aleksandr Litke to the composer, dating from 1891 to 1893, are preserved in the Klin House-Museum Archive.