Sink or Swim by Sarah Mlynowski
Whatever After continues. Read in order, because there are actually two arcs, though mild: what the mirror is doing, and how the fairy tales help Abby adapt to the move.
This one, as you may guess, is the Little Mermaid. (long melodramatic sigh I guess when twisting them, especially for the children, you do have to stick to the Pop Top 20. Though Abby does know the written versions.)
In this one, of course, the written version is quite important, since, as she explains to Jonah once they are drawn into the story, the Little Mermaid dies. For once, they want to meddle.
It involves a servant whose great-grandmother was a mermaid, swim suits, being maid of honor, learning to read and write, a childish nickname, and more.