Star and Twinkle are two bio-mechanical androids hired (or possibly created) by Mama Cosma in one of her plots to end Cosmo and Wanda's marriage.
Mama Cosma wanted Cosmo to dump Wanda and marry either Star or Twinkle because they obeyed both his and her every command, and Mama Cosma never thought that Wanda was right for her son. This plot failed as Cosmo always picked Wanda over them.
These robots are programmed to look better and have a better personality than Wanda. Cosmo may have chosen Wanda over the robots because Cosmo knows Wanda better, or loves her more, or can even be of Cosmo's stupidity.
Star and Twinkle made their debut and only appearance in the show in the episode, Apartnership. They were revealed as robots after Cosmo chose Wanda all over again through a game show. However, Star appeared in the game Nicktoons: Freeze Frame Frenzy, but using the name "Stacey", and the character "The Tooth Fairy" from the same game doesn't bear any resemblance to the actual Tooth Fairy from the show but looks identical to Twinkle, so it could be interpreted that it's actually her and that she was named wrongly like many other characters from the game.