I agree with Bill, but there's a couple of points I disagree with. Outside I often use direct on-camera flash at full power to light group photos, but inside I bounce or use studio lighting. Also I never shoot wider than F2.8 in a church, mainly because that's as fast as my IS lenses go, but also because unless all the subjects are parallel the DOF isn't wide enough. A fairly typical exposure in a church here is ISO1600 F2.8 1/30th - 1/80th. Off camera flash is great, and I use it a lot, but it's not always necessary.
I would like to 2nd what Bill said about hiring a pro. They'll spend huge amounts of money on their wedding day and all they'll have to show for it (apart from being married) is the photos. The difference between a family member taking wedding photos for the first time and an experienced pro is huge. I think photography is one of the most valuable and worthwhile things to invest a wedding budget in, and it really is an investment, because they'll have the photos for the rest of their lives. You owe it to her to explain all this, show her your photos, and show her photos from professionals, and let her make an informed choice.