AmandaLouise, I think your response is quite over the top and judgemental.
Claritytoo, I understand why you felt you needed to pretend to be that alter, and yes, alters within me have done that at times, too. I have certainly had times when the 'wrong alter' was in a particular situation and for various reasons has had to pretend to be another in order to protect the system. What most springs to mind for me is child alters having to pretend to be adult alters in work situations where they have slipped out inappropriately. I totally get that the circumstances that lead to this happening for you (talking your sister in the middle of a therapy session). Your sister alter may have not come out to deal with the phone call because of the therapy situation, and I totally understand that the system as a whole *needed* the sister to remain unaware of your DID, and thus knowingly pretending to sound like her was the most appropriate thing to do in the situation. Here where I live and work that is not considered to be faking in the sense that it casts doubt over your DID and raises questions about you faking the whole thing. Here where I live and work that is considered to be a normal, creative way of protecting the DID system.