C.M.
I wrote down 365 memories my mother and I have and gave them to her in a very nice crystal vase when I got married so that every day for a year she could read a memory about us. I also told her the crystal vase was for candy so when her future grandkids came over, they could have a 'go to' candy dish (as my grandparents had). She took her time and really savored reading those memories and instead of reading 1 per day, every few weeks she'd read a few so it took her 3 years to read through all of them!!
You could get your sister a vase (does she want kids?) and tell her that is for candy for her future kids and then write down memories and put them in there...doesn't have to be 365...but I like that idea!!
Some of my memories were as simple as "Wedding dress shopping" or as extensive as "Remember when I was 16 and blah blah blah." Get started soon as you will find it hard to write down so many memories but then if you try to cram it into one session or a short period of time, you'll remember other memories later and wish you put them into the jar.
You could also do the same thing and put it into a nice flower vase, fruit bowl, etc.
A second idea would be to give her a very nice tree or shrub and ask her if you can plant it together. Tell her you wanted the two of you together to plant something because you know now that she'll be starting her life as a new role - wife, and you'd love to see her 'grow' as a person in that role.
Third idea would be some kind of very nice jewelry that says something about being a sister or twin.
Fourth idea would be to make her a scrapbook so that she can fill it with pics of wedding planning - You can do an 8x8 or 12x12 - or even buy pre-made layouts (more expensive). An even greater idea would be to get pics along the way (dress shopping, shoe shopping, cake shopping, etc - you may need to enlist fiance for this) and get all the pics printed out and put them into the scrapbook yourself and then give it to her at the rehearsal dinner.
Good luck and enjoy the wedding!!