Eek, I hate public speaking too
I think the bullet points is a good idea - you won't feel 'scripted', but you'll have a prompt if you lose your train of thought and get stuck!
I always find the best way to do things like this, is to just focus on one person that you're familiar with - in this case, the happy couple would be great. Don't worry about the rest of your audience, pretend they're not there if you can, and that you're just speaking to your sister about how stoked you are that she's found 'the one' and that you're wishing her a very long and happy life with her husband standing by her side. Good luck! Congrats to her, I hope she has an awesome wedding day!