Warning: SPOILERS for Powers of X #4
Marvel's X-Men reboot has just teased that there may be a fourth Summers brother. The Summers family has always been at the heart of the X-Men franchise. Cyclops and his brother Havok have been central figures in almost every incarnation of the X-Men and X-Factor.
In X-Men #23, writer Fabian Nicieza teased that there may be other Summers siblings. Classic X-Men villain Mr. Sinister made a throwaway reference to his desire to protect Cyclops and his "brothers" from the threat of the Legacy Virus. The dialogue clearly set up a third Summers brother, and perhaps others too. This plot thread came to a head in the X-Men: Deadly Genesis miniseries, which revealed Cyclops' previously-unknown brother, Vulcan, who proved to be something of a big deal when he went on to conquer the Shi'ar Empire. Vulcan is an Omega Level mutant who's returning as part of the X-Men reboot, destined to join the rest of his family in the upcoming X-Men book.
Jonathan Hickman's Powers of X #4 suggests that there may be a fourth Summers brother. It includes a couple of pages of supposed gossip from the Bar Sinister, and one is particularly interesting:
"Two brothers jumped out of a plane, and for the longest time, until he was discovered, many wondered if there was a third. If we told you there were more, would you believe me? Probably not."
This may just be bar-room gossip, but it's important because of its source. Sinister was always obsessed with the Summers genome, which he believed had the potential to unlock the future of the entire mutant race. His obsession appears to have been justified, given the wider Summers family includes a number of Omega level mutants such as Vulcan, Cable, Rachel Grey, and Nate Grey. If anyone knows the secrets of the Summers family clan, then it's Sinister.
This also fits remarkably well with X-Men continuity. The comics have never established a way for Sinister to know about Vulcan, given he was trained by Moira MacTaggert and didn't know his true identity at first. It's frankly more reasonable for there to be yet another Summers brother, one whose history is completely unknown at this stage, but who could conceivably have been in Sinister's hands all this time. Interestingly, back in the '90s Marvel toyed with revealing a number of characters were actually related to Cyclops, including the likes of Adam-X and even Gambit. But presumably Hickman has something else in mind.
Powers of X #4 is on sale now from Marvel Comics.