The pronoun she
could suggest to the reader that Peter is thinking of a woman, perhaps his wife (see 1 Corinthians 9:5). It would seem strange, however, that Peter would refer to his wife in such a roundabout way when he speaks of his son in a very direct way in the very next phrase. More acceptable, then, is the historic understanding that Peter is here referencing the church in Babylon.
13 She who is at Babylon, who is likewise chosen, sends you greetings, and so does Mark, my son.