I came across this post by accident, but have been covering weddings for 20 years on and off. For group shots (and here I have to admit I am a Nikon user but the principle is the same) I use the 17/55 F2.8 lens, which don't forget is really 25/80ish in digital terms anyway. The wide angle is great, and enables some great shots. For later on, for candid portraits and so on, I use the 70/200 F2.8 VRIFED, which is the IS lens in Canon terms. The large glass enables non flash indoor photography if the light is reasonably good, but with a powerful dedicated flash virtually anything is possible from a good distance.
What I would emphasise is the F2.8 throughout on both lenses, a feature of Canon lenses too. For portraiture the ability to create bokeh (subject in focus, background blurred) is essential for those almost studio quality portraits.
I would recommend the Canon 24/70 F2.8 and the 70/200 F2.8 either with or without image stabilisation. I have the IS type lenses for motor racing (for magazines so quality is all important) but I am not sure it is worth paying for it if you are not into action sports. Unfortunately these are expensive pieces of glass, and happily Sigma can accomodate you in this matter with the 28/70 F2.8 and the 70/200 also. They are good lenses and IMHO quite frankly it is only in the laboratory that the difference between Sigma and Canon glass will show. How many people look at their wedding photos with a magnifying glass?
I hope you come to the same conclusion and wish you luck with your friends weddings - you are usually, in my experience, on a hiding to nothing anyway!