Being Matron of Honor means alot of work. I have done it twice. Once well, once not so well.
I will take a page from MY maid of honor 7 years ago, and tell you that without her, I would not have made it. She mailed and e-mailed me pictures of flowers and arrangements, of dresses, colors, anything she saw- she kept a note of. She organized ONE bridal shower, if there were more she attended if at all possible. As an attendee, she wrote down everything I received and who it was from.
She came bridesmaid dress shopping with me and tried on everything I asked, ugly or not. She helped with party favors and ideas, sent me bridal magazines with things circled that she thought I might like, and on the big day, had her mom (who did mary kay VERY well) do my makeup, helped with hair, etc. She planned one night out with the girls (bachelorette) for me, which was dinner, fun lingerie gifts, and a movie. Totally me. I am so not into the wild nights out.
For your sister, get a subscription to a couple of bridal magazines- or pick up a few here and there. Either sit with her or mail her pieces or like my best, the entire mag with pages marked. Plan an outing with her that her ladies can come too and do lunch, try on dresses- no purchase necessary, etc. Take her to see a florist, etc.
Basically, you become the planner and the helper with all the extra bits and pieces. Don't go over board, but be there for any ideas, and be excited with her. This is HUGE for her, and can be overwhelming- for you, too. So get a book for planning- or get her one- and make yourself a copy- my best bought one and filled it out and then gave it to me as a gift- with all the bits in it that we had searched through, tested, etc.
ALSO, take her to cake test. Depending on where you are, there is a fantastic little tiny shop in Bakersfield. It's called L'Amour Cakes. The owner is a bit eccentric, and the shop is a little quirky, bit the cake was phenominal and you can get full size test cakes to take home for $5 each.
The toast- something from your heart. How proud you are of her, how amazing she is, she looks, etc. What a wonderful life you wish the two of them...please no "what to expect from your lazy man" speeches. Men will leave their socks on the floor. Everyone knows this. Keep it positive, keep it focused on her attributes, and how great of a choice she made for a husband, and how lucky they are to get to spend their life with someone they love so much. What a blessing.
I think that's all I've got. Good luck, and have fun!