How about one day one of the sisters, the more adventurous one, comes upon an underwater cave in which she finds ancient mermaid writing. Though the cryptic writing looks familiar, it takes a long while and lots of research in the library's archives to understand the writing. The writing tells of the mermaid history and the prophecy of a set of twin mermaids (them) and how they are destined to fulfill some important mermaid destiny. Just an idea, we don't actually have to do that, lol.
As for the jewelry, maybe it could glow when they come in close contact with something having to do with the mermaids. They can be like aquamarine colored, perhaps. I'm not too sure.