My sister, my sister!’
This does now make me think that the S.O. who wrote this letter is a woman, just because of how similarly her love to Alexandra M. is to Netochka's love for Katya. So I can also draw parallels to Netochka, and the author of this letter.
Alexandra M. is the parallel of Efimov and S.O. is the parallel of Netochka. Hmmm
