Heir to the Empire and Timothy Zhan who gave us Thrawn and Mara Jade, Talon Karrde, the Noghri, and Joruus C"Boath, first created Jaina Solo.
Princess Leia was 23 years old in ROTJ, based on her and Luke being 19 in ANH, four years earlier.
HTTE takes place 5 years after ROTJ, so Leia is 28 when Jaina is born.
Carrie Fisher is about 30 years older (than 19) now (when she filmed ANH, she was younger than 19 I believe - younger than the character of Princess Leia she portrayed).
Thus, Jaina is about 30 years old in comparison to Princess Leia, were both to appear in the same movie.
Mara Jade-Skywalker is pregnant with Ben Skywalker during the Yuuzhan Vong Invasion Wars. They are set 25 years ABY (when the first Death Star was destroyed in ANH, which is 4 years before ROTJ).
So that's 21 years after ROTJ when Luke is 23. Thus Luke becomes a Dad when he is 44 years old.
Mark is now over 60 I believe, but since he and Carrie play twins, we must regress him to her age (Lucky Mark!).
So doing the math, Ben Skywalker would be 15 years old (approximately) in a new SW movie with Luke being about 60 years old.
Jaina is twice Ben's age at this point: 30 and 15, respectively for the cousins.
If I'm not mistaken, she is psychologically scarred from having to kill her own brother (Darth Caedus) Jacen Solo by this time (Anakin Solo died during the war).
Her neice (Jacen's daughter with Tenel Ka) - Alanah Solo, raised by Han and Leia (grandparents) - would be a toddler.
Sequel movies could advance Ben Skywalker 5 years or more, into his 20's by the 2nd or 3rd film.
Alanah Solo might be his responsibility to train his cousin?
The same actress could portray Jaina Solo in her late 20's to mid-30's throughout the next trilogy.
THERE HAS BEEN TENS OF THOUSANDS OF DOLLARS - MAYBE HUNDREDS - INVESTED IN A SINGLE CHRONOLOGY FOR STAR WARS IN THE EXPANDED UNIVERSE.
Careful records of the timeline have been kept (though admittedly violated when it's convenient), but NO ONE WILL BUY HUNDREDS OF THESE BOOKS ANY LONGER if the new movies invalidate the EXPANDED UNIVERSE.
- or the action figures of Thrawn, Mara Jade, etc.
So I for one hope they hold the Expanded Universe's integrity together for the most part.
And while not every book has been good, and some were awful, not every episode of the Clone Wars - or even the live-action movies were great either.