If you and your sister have a warm relationship - maybe a long letter telling her how much you care for her and some memories that are meaningful?
If you and your sister are not close - maybe a check or cash telling her that it's a gift certificate to her favorite store or to apply it to something she has been wanting.
If she has way more money than you, than just find an appropriate card, show up at the party (if you want to) - if you don't plan to go, please send regrets - and let it go at that.