For any event photography I use 17-35 2.8 and 80-200 2.8 with full frame camera.

Good luck.

How much do you have to spend?

I agree with Matt - I use(d) the 70-200 for the majority of close up portraits and in the church (with a tripod) to get close to the front without actually being at the front of the church.  The 24-70 for wider portraits or group shots, and a 16-35 for some wide angle love.  If you can afford a 24-70 f/2.8 type lens it is a nice all-around lens for portraits and reception.

I explained to my sister in law yesterday I don't have the equipment for weddings and she still wants me to do and said that her niece would more than likely buy me the lens I needed, but I doubt she'll give me the price of the 24-70. That would be to good to be true.  Even used they aren't cheep. I don't even know how the lighting is in the church. I live in Alabama...the wedding is in Illinois, so I can't go check it out first. I know the reception hall I've been to and it is not real bright. She said she would make sure I was seated up front...so I don't think length of the lens will be a big factor. I have the 18-200 but don't really care for it's lack of sharpness.

I'm going to go look on KEH Photo and see what their prices run for the 70-200 and the 16-35. I have a 50mm...would that do for the portrait shots?

Just don't do this!!!

http://www.videobash.com/video_show/wedding-photography-fail-4977

She said she would make sure I was seated up front...so I don't think length of the lens will be a big factor.

Seated?  You should consider calling wedding photographers local to you and asking them if you can haul their gear around during their coverage of a wedding.  Basically be a fly on the wall and learn as much as you can as fast as you can.  

If you intend to provide good coverage you will need to be moving around.  Find good angles, get the Brides face and the Grooms face in different shots.  If you stay seated you will have one angle for the entire ceremony.  If that's the case then you had darn well better make sure it is a great angle.

Rent fast lenses.  Lensprotogo.com  Rent fast lenses.

Good luck, my fingers are crossed on your behalf.

Mat

Why don't you call the church directly and explain your situation.  Some priests think the photographer is just as important as the bride - others have rigid rules.  Regardless I don't think the pastor will expect the offical photographer to sit in a seat for the whole ceremony.   Also, good to know if you can use flash at certain points.  I've even had a pastor ask ME if it was okay for the congregation to take flash photos during the ceremony, yikes! 

If you can make it to the rehearsal the night before you will feel worlds better, you will know which stages are important and will know where you want to be for them and can even discuss with the priest if you are okay in that spot (but light won't be the same).

If your neice thinks you will be sitting in a seat during the service, obviously she isn't expecting a lot.  Some nice photos of the couple in front of the church and in the park afterward will likely be more meaningful.  Especially if great grandparents. etc. can stay for a few shots.