Hi Tina77
You can ring the number on the assessment letter and change the appointment. However, you can only do this once. If she was to try and change it again her papers will be returned to the DWP and there is a risk her claim can be closed.
If you have dates when your niece did not want an assessment these should have been added to her form. There may be very little flexibility and I doubt very much you can try and say holidays - as that will be the Easter Holidays and too far away. All you/she can do - is ring and ask.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
