Even by the considerable standards of the X-Men, Cyclops has an extraordinarily complicated family history. In addition to cloned wives and children from the future, Scott also has at least one sibling he never even knew existed, if not two, and the mystery of the missing Summers Brother(s) was one of the most enduring X-Men sub-plots of the '90s. And now that the new miniseries X-Men Legends is set to revisit that sub-plot, we're taking a quick look back at how it came to be.
Early on in his time with the X-Men, Cyclops reunited with his brother Alex, who would later join the team as Havok. When he was a child, Scott's family was on a plane that exploded. His parents threw him and Alex out with the only parachute on board and were presumed dead. The boys were separated while in foster care, at which time they were targeted by Mister Sinister, who experimented on Scott and tampered with both of their memories. As Cyclops, Scott would later encounter Corsair, leader of the space pirates known as the Starjammers, and learn that he was really his father, who had survived the accident and been abducted by the aliens of the Shi'ar Empire. Eventually, Scott learned that Corsair was his father, who had survived the accident, which in reality was an attack by the alien Shi'ar Empire.
Later on, an encounter with Sinister would further complicate the Summers family when the villain accidentally suggested to Scott that he actually had more than one brother. In 1993's X-Men #23, by Fabian Nicieza, Andy Kubert, Mark Pennington, Joe Rosas, and Bill Oakley, Sinister mentions Scott's "brothers," with Scott noting how strange it was for him to use the plural.
The possibility of one or more additional Summers brothers existing haunted the X-titles for several years afterward. Adam X, an occasional ally to the X-Men also known as Adam X the X-Treme, was implied to be related to the Summers. He once met with Scott's grandfather, Philip, and Jean Grey sensed a connection between him and the family. Fabian Nicieza has since confirmed that he did in fact intend for Adam to be revealed as the long-lost half-brother of Cyclops and Havok.
At the time, there were also hints that Gambit could be the third brother early on in his time with the X-Men when his history was still mostly unknown. Gambit's brown hair and energy-based powers are notably similar to Scott's and he also has a history with Sinister. Chris Claremont and Sena Chen's X-Men: The End miniseries, set in an alternate future, revealed that Gambit had been created by Sinister using both his own DNA and Scott's, making him a relation of some sort of Scott and Alex's and explaining Sinister's original hint at the third brother. Of course, the Gambit idea was soon abandoned in main continuity when his real backstory was developed.
After the subplot faded away as the '90s came to an end, the idea of the Third Summers Brother returned with a vengeance when Vulcan, the official Third Summers Brother, was introduced in 2006.
Created by Ed Brubaker and Trevor Hairsine, Vulcan first appeared in X-Men: Deadly Genesis #1 in 2006. The Deadly Genesis miniseries revealed that Katherine Summers had been pregnant with the child who would eventually be known as Gabriel when the Shi'ar attacked the Summers' plane. After D'Ken killed Katherine, the fetus was removed from her body and grown to adolescence in an incubation-accelerator. Gabriel was sent to Earth to serve Erik the Red before being found by Moira MacTaggert.
When Krakoa the living island, then an enemy of the X-Men, took the original team captive, Gabriel, who called himself Kid Vulcan, was recruited onto the first rescue team Professor X assembled, prior to the famous All-New All-Different X-Men. Deadly Genesis revealed that the original rescue team's mission was a disaster, with all members presumed dead.
Gabriel had survived, however, and absorbed the powers of his teammates, Sway, Petra and Darwin, with the latter's abilities allowing him to survive in a hibernating state when Polaris sent Krakoa into space. Gabriel was awakened by mutant energy released on M-Day and returned to Earth as Vulcan, seeking revenge against Professor X. It is also revealed that the Professor erased Scott's knowledge of Gabriel, seeking to spare him the pain of losing his brother.
Gabriel was revived at the start of the Dawn of X era, and like many former mutant villains, now lives in harmony with the other mutants of Krakoa. He is shown to have healed his relationships with his brothers and the rest of the extended Summers family and lives with them on the Moon. When the Cotati were preparing to invade Krakoa during the Empyre crossover, Gabriel remembered repressed memories from immediately after his battle with Black Bolt, in which a group of unknown beings attempted to free an evil side of his personality.
While Vulcan may be surrounded by questions about his own current state, X-Men: Legends is set to finally put the mystery of Adam X's connection to Cyclops and Havok to rest after all these years.