proofreader here: My bad, I couldn't pay much attention to this chapter. probably shouldve left a note or two in that page but for better understanding you could read it as, "if you treat me like your sister right now, this tomboyish side of me (she used the pronoun boku here which if u dont know is now a days mostly used by tomboys and teenage boys) will become a real girl." if you don't get what real girl means here in this context here, shes trying to convey that she will start to like Hayato like a real girl instead of their friendship where Hayato used to think she was a boy.
hope this explains it!