Of Course yes,just be confident enough,take it up as a challenge if you are nervous of people looking at you and just waiting to notice your mistakes just be clam,do not show them that your nervous..Just be yourself..You could start with a small introduction,an experience of you n your sister before the evening,and tell your sister that she is looking very beautiful and how much she and her happiness means to you...let it flow from your heart.keep it short and end with an emotional punch line...Confidence is all that matters,keep a smiling face and it will all go well..
Wish you luck
Best speech ever? Cliche, speak from the heart...good luck
You should just tell her how you feel. And let it come from the heart.