Third Summers Brother

The "Third Summers Brothers" was a plot point in the X-Men comics that wasn't resolve until 13 years later.
Mr. Sinister's Reveal
The genetics-obsessed Mr. Sinister and Cyclops were talking. Mr. Sinister mentioned Scott's "brothers", but Cyclops only has one brother (Alex). Despite Sinister claiming he just misspoke, people began to speculate on who the third brother was.
Failed Attempts
Fans began to wonder who the third brother was. Several writer tried to resolve the plot line, but ultimately failed. Among the intended brothers were:

• Adam X is a half-human and half-alien. Adam's human parent was suppose to be  Kate Summers (Cyclops' mom).
• Apocalypse was suppose to be the illegitimate son of Chris Summers (Cyclop's father) and an unknown woman. A time traveller would kidnap the future Apocalypse and send him to ancient Egypt (this makes more sense if you know his origin) thus making the Summers family the origin of the X-gene. This was abandoned due to how overly complex and stupid this was.

Fans suspected other character as the third brother (such as Gambit due to his energy-based powers and ties to Mr. Sinister) despite the writers not intended as such.
It is... Vulcan! Wait, who?
It was a reveal that a previously non-existing character Vulcan was the last Summers brother.So yes, they had to make up a character to end this story. Vulcan was a half-human and alien. However, there could be a fourth Summers brother since Sinister just says "brothers" and not any actual number.
References:
http://en.wikipedia.org/wiki/Third_Summers_brother