A graduation celebration turned into a disaster this past Saturday night as a brother and sister were killed at a bonfire explosion in the 6500 block of Gratiot Road in St. Clair Township.  Savannah Blewett, 18, and her older brother, Christopher, 27, died when a two-story bonfire was doused with gasoline and then lit by the 18 year old girl, exploded.  Police were called to the scene by a neighbor shortly before 9:30 p.m. according to a news release from the St. Clair County Sheriff Department.  The explosion caused wood to fly up to 100 yards away and the windows in the back of the residence more than 100 feet away blew out.  Three others were taken to area hospitals with undisclosed injuries, and another person was treated at the scene.  The incident remains under investigation.