Hindu, Sikh, Muslim? Google 'Hindu wedding' , 'Sikh wedding' etc. Loads of information explaining the ceremonies and what kind of dress code to expect. Modesty is the watchword whatever the particular religion however, so lots of bare flesh on display not a good idea! Take your cue from the other female guests.
I attended an Indian (Muslim) wedding a couple of years ago when one of my then collegues got married. There was only a handful of non-Muslims there and most of us wore trousers. Thouroughly enjoyed the experience, although quite different from any other wedding I have been to! No alcohol obviously but the food was amaing!!
I went to a Hindu wedding and was practically ordered to wear bright, colourful clothes! 'Twas a good excuse to raid the clothes shops in the Indian part of town!
This might not be that good an example - but I remember Dev and Sunita's wedding in Corrie, and all of the 'non-Indian' guests just wore what they would wear to any other wedding - suits and hats etc...
The above answers are spot on. Elegance is the key. A smart colourful suit for ladies will go down very well. Dont be shy. Wear your best 'going to the races' hat. Also dont reveal too much cleavage. If you really want to put others in the shade, wear a sari (borrow it from a friend if you must). A colourful sari on a non indian person looks great methinks.