Jilly and the Jade Journey (Crystal Cove) -

Jilly usually finds being a Fin Friend very easy, but there’s one merfriend they just can’t get along with: Joy, their new step-sibling. But when one of Joy’s many tantrums leads to a huge magical discovery, Jilly starts to wonder if they’ve got their step-sibling all wrong. What if there’s more to Joy than they first thought?