Feeling bad must be difficult for you! However, she's your sister so try to rise to the occasion and share her happiness with her as much and as strongly as you possibly can. You will not have another chance at celebrating her new marriage and all the wedding excitment that goes with it, and she may pick up your feelings..and if that happens you will feel awful about the whole thing and not have a second chance at sharing in such a sacred, special time. Family truly is everything, don't let her down.
just try and do your best. shell know you care. it sounds kind of exciting to me.