Five Night's at Candy's Sister Location is a game and the speculated 4th installment of the Five Nights at Candy's franchise. The game has not been created yet.
Over at this reddit post, Emil comfirmed, that he indeed is working on a fan made sister location and might at some point release his own.
Some fans of the series are in great disbelief, that the game will happen, while others are eagerly awating it and spawning fan theories about it.
Despite this, there is still a lot of speculation involved, so we have no idea, if the game will come to be or not. We may not know, but keep your hopes up high.