Little Mermaid: Reflections of Arsulu

This one's got a pretty basic plot, but it also has the most appearances of the sisters other than Ariel. Ariel's pretty much been the focus of the first two books, but while she takes the lead in solving the problem, she shares the spotlight far more equally with her sisters.

Adella befriends a new mermaid named Arsulu, who is searching for her lost betrothed. Arsulu seems very sweet and everyone likes her, but not long after her arrival, the sisters are all at each other's throats. Only Ariel is suspicious and it's she who discovers that Arsulu is Ursula in disguise and of course, it's Ariel who saves the day.

The best part of this book is learning some more sister facts!

Alana is really good with sea creatures.

Aquata takes ballet class. 

Arista took a cooking course.

Andrina's a bit wild and also rides sea horses like Arista.

Attina loves reading.

Adella is very quick to think the source of every issue is envy.