Sister Sonic is a cancelled localization of Popful Mail announced sometime in 1993.
Originally, the plan was to replace the existing cast of the game with Sonic characters, including Sonic's long lost sister, Sister Sonic. Due to a large number of angry letters sent to Sega Falcom (the name which would be used for the Sega and Falcom team) it was decided to cancel this port and instead settle on a more direct port of Popful Mail to the Mega CD. This would have been the first RPG in the Sonic the Hedgehog series.
Because of this game's cancellation, the Sonic franchise would not feature a RPG until 2008 when Sonic Chronicles: The Dark Brotherhood was developed by Sega and Bioware. According to Electronic Gaming Monthly, this scenario was a great example of the consumer's power to influence a video game company's decisions.