[Probably too late on this, but anyhoo...]
On a more serious note I think your skills are well up to the challenge but the difficult part is going to be balancing the photography with your bridesmaidal duties. Unless you can coerce someone to be your assistant for the day I'd keep it real simple - just the D7000 + the 18-55mm zoom + external flash (if required) mounted on camera. Despite the zombieness the usual 'rules' of photography still apply - e.g. try to avoid harsh direct sunlight, etc.
If using flash try bouncing flash of the ceiling if it's neutral-coloured and not too high. I only ever shoot events in which I'm also 'participating' (
) so I set the camera to do the 'heavy lifting' (as McNally would say) which allows quick shooting and not having to worry about settings for each shot. With TTL and auto-ISO enabled typical settings of 1/60 and f/5 work well for me. (That's using an SB-800 on a D300, I'm not sure if your Sigma will work the same way). Adjusting the shutter-speed will vary the amount of ambient light you capture (but obviously a longer s/s may lead to blurring).
You mentioned some surprises so it might be good to co-ordinate with another photographer so that one of you captures what's happening and the other captures the guests reaction to it.
If you get a chance maybe shoot the detail images (dress/costume, etc) today and get them uploaded and 'out of the way' to save some time tomorrow.
Don't forget to charge batteries and clear memory cards (and pack spares if you have them).
Shoot lots (when/if you can).
Most importantly enjoy the day. (Don't let your photo 'duties' get in the way of sharing your sister's special day...)
And congrats on the impending photo opportunity...
Good luck.