Gambit
Several hints were made to Gambit being a third-Summers brother shortly after his introduction into the comics. His most notable similarities were to Cyclops, as both of them shared dark brown hair and red eyes. Gambit's origins prior to being adopted by the Thieves Guild were completely unknown, causing much speculation as to him possibly being the third Summer's boy. Finally, in the 12 issue, 3 part X-Men: The End series, which presented one possible future of the X-Men, established that Gambit was a genetically engineered mutant, created by Mister Sinister with his DNA spliced partially from Cyclops and Sinister himself, thus making him a third brother in some respects and explaining how Sinister was already aware of there being another Summers child. None of this has been established in the main Marvel continuity at this time.
During an interview with Comic Book Resources, at the question "what's your opinion of the revelation of Gambit being the character in question over in X-Men: The End?", Fabian Nicieza answers that "My opinion is that I screwed up plenty of Chris's stories; he's more than welcome to screw up one of mine."
Read more about this topic: Third Summers Brother