Amateur Wedding Photography

I am writing this message for my sister, who has volunteered to
photograph our niece's wedding.  She is a marvelous amateur
photographer, uses a Canon Digital Rebel and gets amazing shots.  The
niece is paying for the wedding herself and is on a tight budget, so
my sister was glad to offer her services free of charge.  She feels
confident she can photograph all the outdoor shots, reception, etc.,
but is a little nervous about the actual church photography, largely
because the wedding will be at 6PM and so the church lighting has her
a little concerned.  She is open to renting or borrowing or purchasing
any lighting equipment she may need.  I guess she could use  any
helpful words of advice from seasoned photographers.  I am writing
this message in earnest, and surely would appreciate sincere
responses, as I have no wish to start any flame wars, etc.  Thanks in
advance for any 'light' you can shed.