Can I throw a shower for my sister if she's already been married and is renewing her vows after one year?

My sister was married in a courthouse ceremony last June. No one from our family was there, as she didn't want anyone going through the trouble of it. Since she and her husband never had anything to celebrate, they're renewing their vows on their one year anniversary and having a reception following. It's being treated just like an informal wedding in that she's asked me to be a "maid-of-honor" of sorts, and her husband has a best man. Since she never had one, is it improper of me to throw a shower-type luncheon for her with all the women who will be invited to the wedding? My mom said she's never heard of anything like that, but I don't want my sister to miss out on this tradition. It wouldn't be anything fancy, and I had a blast at my shower. Help!