we can not diagnose whether you are DID or not. what we can tell you is that the diagnostic criteria (rules that mental health therapists, doctors, psychiatrists) go by in the USA to diagnose whether a person in general has DID is that the problem can not be because of having imaginary friends, fantasy play.
so .........if going on my location....... the fact that you on purpose created an imaginary person known as your twin brother would knock out the diagnosis of DID. Here in my location someone having an imaginary person in their life is many things but not DID.
examples...
normal
role playing
playing pretend
fantasizing
here in america many normal non mental disordered children have imaginary friends, most that I know are only children who always wanted a brother or sister, others that I know their imaginary friends,brothers, sisters came from their being bored, for attention and still others due to school or acting classes where one of the requirements focused on pretending\roleplaying the part of having an imaginary sibling.
here in my location when having an imaginary sibling negatively affects a persons life, the person who willfully created /pretended they have an imaginary sibling can take control and stop their self, just like stopping another annoying habit like cracking ones fingers, smoking, swearing, twirling hair sucking ones thumb....

my suggestion is when you notice you are talking and interacting with your imaginary brother or are pretending to be your imaginary brother stop yourself and be honest with others around you. example in therapy when pretending to be your imaginary brother you can say something like sorry to confuse you I was just pretending to be my imaginary twin brother. I do not have an alternate personality, then tell your therapist how you came about creating your imaginary twin brother and how having this imaginary twin brother is affecting you and your life. your therapist will most likely understand therapists deal with people playing pretend, role playing all the time and in many cases actually pretending\role playing in therapy can ge quite helpful and healing when a therapist knows its happening. when they dont realize its happening many things can happen like getting misdiagnosed. being hospitalized for psychosis\ depression\ some I know who kept their pretending a secret ended up being given the wrong medication and ECT (shock therapy) and others were diagnosed with a mental disorder called fictitious disorder imposed on self (which means a person is purposely trying to make their self seem to have mental disorders that they do not have)
my suggestion come clean with your therapist and your friends so that you can begin to get rid of this habit of pretending you have a twin brother, that is affecting your life. you may find that you no longer need to pretend you have this imaginary brother anymore.