yes that is a surprising question isnt it. Im guessing your T wants to know how and why Steven perceives "himself" as a male, my suggestion is since you have co consciousness (otherwise you would not know any information about the alters) ask Steven to tell you or write down or tell the therapist how and why he became male, how and why he perceives himself as male and what his sense of agency is.

as for the alter that grew up and didnt go away. alters with DID do not go away, die and other such things. when their sense of agency is no longer needed to be kept separate they integrate \become one whole person with the one they reside with in (body born or alter), since she did not become one whole person with the body born or another alter Im guessing her sense of agency is still needed for everyones survival.

my suggestion is ask her why she did not become one with an alter or the body born/ physical body person.