The rumor comes from schmoesknow.com and links Episode 7 with some characters from the Expanded Universe (the kids of Leia, Han and Luke). Take it with a huge grain of salt but still it's an Episode 7 rumor so read on...
Quote from schmoesknow.com:
A reliable source (who wishes to remain anonymous) has seen a copy of the alleged casting breakdown that has been (or is about to be) sent out from JJ Abrams. That’s right, this supposedly comes straight from the man himself!
Storyline: Two 17 year old twins, a girl and a boy, are trained by their uncle Luke to be the greatest Jedi’s in the galaxy. Problems arise when the male twin turns to the dark side.
Jaina Solo – Female, 17 years old, lead. – Jacen’s twin sister, one of the greatest Jedi’s in the galaxy, trained by her uncle Luke Skywalker
Jacen Solo – Male, 17 years old, lead – Jaina’s twin brother, one of the greatest Jedi’s in the world. A loner who is constantly struggling with the idea of what’s right and what’s wrong. Starts to turn to the dark side. Full of conviction that what he is doing is the “right thing”. Will eventually become Darth Caedus.
Assuming, of course, that this is all valid (which it certainly seems to be), then we now know that Episode VII (and, I’d assume, the rest of the trilogy) will tell the story of Han and Leia’s twins Jacen and Jaina with the focus being on Jacen’s eventual turn to the dark side. We’ve been wondering if these films were going to borrow from the Expanded Universe and it appears we have our answer.