I think I would go in first and try a few shots without flash OR strobe. Depending on how the lighting they have is deployed it may provide enough to give some facial features, but you still retain the semi-dark effect with the christmas lights and sparkles.
I would go in trying ISO 800 or 1200 in Program mode with a -2 or even -3ev adjust.
From there you may have to play it by ear.
I had a situation at christmas where our church sang "silent night" with only candles (about 250 candles). I could have flashed the whole thing, upset everyone and had a nice photo of people in the church all holding something and you cannot tell what they are holding.
I don't remember the settings exactly, but I think it was about ISO1200 and it was a D90 so not great low-light -- the photo was great, but I had time for a few with different settings.
You may then want to try with some flash, perhaps with the flash output lowered 2 or 3ev so you still get SOME sparkles, but you see people - people like to see people at weddings.
From there -- wee what you get -- you probably won't know much until your camera actually sees it. At least you should have a few minutes to play around without losing some good shots.