Francis
I'm afraid I am not surprised that your sister's Psychiatrist did not respond, even though you are close family you have no right to see her medical information without her explicit permission.
Would her GP be willing to write a letter confirming that her conditions are worse than she says they are?
Apart from her not recognising her conditions is your sister capable, if not then you could apply to be an appointee, this would allow you to speak on her behalf, otherwise all you can do is try and add comment during the assessment, but be aware that the assessor will want to hear from your sister and they may ask you to be quiet during the assessment.
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
