I think if you don't tell her and back out now you are going to have a WAY bigger problem on your hands. I know brides can get a little testy but, you are her aunt for goodness sake. She shouldn't treat a professional like that let alone a family member that is doing it FREE OF CHARGE!!!
Weddings are a huge deal and I think you just tell her that exactly what you said here. You don't have the expertise to do the job she is requiring you do. Tell her you would be happy to be around and shoot pictures but that she should look for somebody with the qualifications she is looking for. Only a seasoned professional could handle those high demands and honestly a photographer that has more than one shooter available.
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