Here at the Tau Omega Chapter we reveal big/lil sisters by giving our littles a poem about a day before revelation, and then we read have of it and they respond with the other half. I heard of an interesting way to do it from our new chapter in Miami though, and that was to give each of the sisters a string that was connected to their big/lil, and then they had to find each other through the mess. So I don't know what will work for you, but there are some ideas.
Tau Omega Chapter
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