Third Summers brother

The third Summers brother is a largely speculative plot point launched by 1993's X-Men (vol. 2) #23. In that issue Mister Sinister has a conversation with Cyclops and casually mentions Cyclops' brothers. Cyclops notes that it is odd to talk about his brothers in the plural, since as far as he knows he has just one brother (Alex Summers, also known as Havok).

While Sinister himself immediately claimed that it was just a common mistake of his, the speculation about the mysterious third Summers brother ran rampant among fandom. Gambit and Adam X were both believed to be the missing brother at some point.

The second X-Men: The End series, arguably out of continuity, established that Gambit was the third brother.

The 2006 series X-Men: Deadly Genesis established however that the third Summers brother did indeed exist and was the previously unknown character known as Vulcan. But it is not yet known whether Mister Sinister was even aware of Vulcan's existence, so the link between Vulcan and Mister Sinister's statement is still speculative. It is also pointed out by fans that Mr. Sinister's comment meant there was at least a third brother, he did not specify a number and therefore other brothers might also exist.

Inconsistencies
Marvel continuity historically dictated that siblings could not use their mutant powers against each other. This was used as a plot device in several stories with Cyclops and Havok, and is also seen in storylines with Banshee and Black Tom Cassidy. However, Vulcan was apparently able to use his powers against Scott with no problems. poooooooooooooooooooo