Vulcan
The 2006 series X-Men: Deadly Genesis established that a third Summers brother did indeed exist and was the previously unknown character Vulcan. The series sales was not good at the beginning. During an interview with IGN, the editor Nick Lowe declares that the information about the presence of the third Summers brother in the story was revealed to increase the sales.
It is not yet known whether Mister Sinister was even aware of Vulcan's existence, particularly since at the time he spoke to Cyclops about having more than one brother, Vulcan was believed to have been dead for several years with no evidence to the contrary. As such, the link between Vulcan and Mister Sinister's statement is still speculative. It is also pointed out by fans that Mister Sinister's comment meant there was at least a third brother, he did not specify a number and therefore other brothers might also exist. In any case, the current version of the story could negate the scenario of Adam X being the son of D'Ken and Katherine Summers, as she was already pregnant when they met, and died before Adam X was born, though this could be excused by the possibility that Adam-X is a test tube baby, and it has been shown that in-vitro fertilization and ectogenesis are common Shi'ar practices.
Read more about this topic: Third Summers Brother