Originally posted by dave_roe . . .
Be honest and just tell her what your feeling are.
and don't forget, she is not the only one that must be kept happy with your results
her future spouse, in laws and other relatives may not be happy with the results despite your best efforts on her behalf
I would urge you, at a minimum, to discuss it with her and her future husband
if you feel this way, be sure to indicate you have questions about how you are to enjoy your sister's wedding reception if you are expected to act as photographer.
myself, I would feel that I am being asked not to enjoy the occasion with her and her new husband and other people but to work it
being the family photographer along with the professional is the best route but then you want to work closely with the pro and do nothing to make that job more difficult