Arista is Triton's fifth daughter thus one of Ariel's older sisters, she is in the original movie as well as pretty much every other Little Mermaid media from the sequel, the prequel, the cartoon show, the books, the comics, etc, she even appears in Kingdom Hearts 3. The only relevant media she's completely absent from is the live action remake, and that's only because all of Ariel's sisters get rewritten, redesigned, and renamed.