They do resemble, but it's not this artist's fault as they're both background ponies. Amethyst Star/Sparkler is a bit purpler than Sea Swirl/Seafoam, though. Observe. (This is from "The Mysterious Mare-Do-Well." A lot of missing CMs. They're standing side by side to the left of the brown pony; you can see that they're similar but not identical.)And Sparkler and Dinky have been known to be sisters ever since "Sisterhooves Social."