Didn't Kairi make the Oathkeeper charm in the beggining of KH1? She was making something out of shells that sailors used as "good luck." and when she gave Sora the star, she said it was her "lucky charm." I don't think it had the same implications as paupou fruit. Correct me if I'm wrong.
They're made from Thallassa, no connection to the paupu.
Who knows,
They might be related, but then Kairi would remmeber having a big sister.
If she did, Kairi wouldn't say. Besides, KAiri confirmed she didn't remember in KH1.