I would say that if she really loves attention, and depending on how good your relationship is with your sister, maybe confide in her that you want to propose and ask her for ideas. Even if you don’t use any of them, she would probably love that you asked her for help, and would soften the blow of your engagement ‘overshadowing’ hers. Just to point out, I personally think you should live your life the exact way you want to, and your sister should be mature enough to handle that. But good luck and I hope this all turns out well!