Not being familar with them I would probably ask a member of in laws family to help. Is there are aunt/cousin who might be able to help? They could be very flattered that you consider them experts on all things sari.
SIL to be doesn't actually have any extended family here - she's adopted, and she's giving all of us exactly the same advice - "They'll tell you what suits you in the shop - you'll look lovely, really" - I think she's trying to keep everyone happy!
They may tell you in the shop what suits you but how are you going to get dressed on the day? I had a friend who was given a sari by her boyfriend but she used to get an Indian friend to come round to dress her when she wore it. It seemed to be hellishly difficult for her to attempt it herself!
My Mums best friend is Bangladeshi, but she's conveniently taken herself off on a three month trip to visit family <<hmmph, the inconsideration>> so I guess I'll have to throw myself on the mercy of the dress shop.
I'm going to be like a sparkly chiffon Andrex puppy, aren't I?
look on youtube, they have some vids. I had to wear one for the first time last year as part of school festival and proper sari-wearers were very impressed by my efforts. You have to tie the petticoat thing very tightly around your tummy, and take time to make sure the bottom is level.
I vote for throwing yourself on the mercy of the dress shop. You could ask if they are open on the day of the wedding and if they would dress you? I did this once with some friends, we arranged it so we would all donate to a charity they collected for if they (the shop) would come round and dress us. Absolutley no way I could have done it myself without spending every day in there practising!
If you go to the sari shop, ask them about ready made saris. I bought one of these in India last year. Basically, its a 'skirt' and a 'blouse' (2 separate pieces iyswim) and a scarf that you wear over your shoulder and down the front. They arent traditional but would be perfect for a novice sari wearer. I got lots of compliments off dp's family because I had managed to find one. HTH
My DH is Indian and I wore a 'lengha' for the wedding, which was really easy to wear; it's a long skirt and blouse top and comes with a piece of material that can be draped and pinned so that it looks like you're wearing a sari. Perhaps ask your SIL if you could wear a lengha?
My bridesmaids (and my very English mum!) wore saris that I brought back from India. There is no way you'll be able to tie your own sari unless, as mumof2teenboys suggested, you buy one that is partly stitched - most Indian women need someone to help them tie one from scratch, not to mention using tonnes of safety pins too to keep it in place!
If you do want to wear a sari rather than a lengha, perhaps you can ask to borrow one as they are 'one size fits all' - you just wrap it round more/fold it under, depending on your size. They can be very expensive to buy because of all the sparkly bits and you'll probably never wear it again.
One final alternative (particularly if you're not too comfortable about your figure) is to wear a 'shalwar kameez', which is a tunic top and trousers and very comfortable. They don't look as formal as saris but ask your SIL what she thinks.