A Sonic the Hedgehog video game
that never existed beyond the drawing board: that's all Sister Sonic
will ever be.
In 1992 when Sega's Sonic the Hedgehog was at the height of his popularity Sega wanted to produce a Sonic RPG to capitalize on the popularity of their hedgehog and that of role playing games. Their plan was to create a new character - Sonic the Hedgehog's previously unseen sister - and make her the main character of their game.
The game was to topline the then-new Sega CD machine and feature characters and realms from past Sonic games. However, rather than create a new game from scratch, Sega planned to take the popular action/RPG title Popful Mail and replace all the characters and locations with the new Sister Sonic concept. That's when things began to sour.
Popful Mail fans, naturally, hated this idea. Really hated it. Fans flooded Sega offices with begs and threats to not tamper with Popful Mail and leave the game alone. Sega saw how the tide was turning and brought out Popful Mail in its original form. The game then went on to become one of the most popular games on the failing system.
The Sister Sonic concept was never revived, although one piece of it lived on. The character art for the pink female hedgehog was eventually revived and converted into Sonic the Hedgehog's girlfriend Amy Rose who later went on to star in Sonic CD, Sonic Adventure, Sonic Adventure 2, Sonic Adventure 2 Battle, Sonic Advance, and Sonic Advance 2.
Info from what I remember of the whole fiasco back in 1992 and from http://www.sonichq.org/games/index/popful/