For starters, CONGRATULATIONS on taking quota plus, especially with your sister!
I've never been fortunate enough to have a biological sister, but we initiated three legacies last term. I'll ask them if they did anything special.
My own ideas would depend on how much you want to spend. One thing that I have used more as both an active and an alumna is a pocket-sized pin holder. The big ones are nice, but in a hurry, the small one is perfect. This isn't mine, but similar:

It's $17.75 on
Gift Mart. Mine is yellow gold, with the crest and no stones on it. I could tell you the importance of the pocket sized pinbox, but I'd have to kill you!

This is also available through Gift Mart, and the two hearts is a very nice way of expressing sisterly love.
Another idea would be to find matching rings or such. They start at $23.00 on Gift Mart. Or, you could go to Things Remembered, find something that you know she'd like, and have it engraved. But I think I'd go with the two of whatevers.