The Little Mermaid is the story of one mermaid, Ariel, who possesses an entire cave full of bullsh*t and all of her father's attention and adoration. What The Little Mermaid isn't is the story of these six nobodies.
Lest we forget, The Little Mermaid starts by informing us that Ariel is one of many, many King Triton daughters.
Oh, we are the daughters of Triton
Great father who loves us and named us well
And then there is the youngest in her musical debut
A seventh little sister, we're presenting her to you
To sing a song Sebastian wrote, her voice is like a bell
She's our sister Ari--*gasp* *oh snap* *this b*tch* (presumably)
And that's it. They only pop up again to talk about Ariel. In the musical version at least they get a song about it. But the musical also makes a very strong point of telling us what we already knew: Triton only loves Ariel and the rest are multicolored trash in his eyes. Basically. That's the basic implication.
On top of the general sense that daddy ignores them 24/7 in favor of ole purpleboobs over here, we are also told that the sisters and Sebastian rehearsed their big number and Ariel -- the star of said performance -- never even bothered showing up to rehearsal. My inner theater kid is OUTRAGED.
To you, Triton's other daughters, you poor unfortunate souls, we wish you well. Now get out of the way so the pretty one can sing.