I think a tasteful cocktail dress would be appropriate.
edit
My 50 year old aunt just worse a satin wrap blouse with well tailored black trousers to a 5pm wedding last weekend. She looked gorgeous and appropriate She paired it with gold kitten heel shoes (her wrap blouse was goldy colored) and pretty gold jewelry.
If you'd like to try new things:
This asian style formal dress is often adapted for middle aged women in big celebrations including wedding
The dress is usually very conservative, so that might suit you.