Jacen knew far more about Jaina and Tenel Ka than they knew about each other, and they had told him a great deal from their conversation from what they had said, and just as important, what had not been said. Darth Caedus, the new Sith Lord he was supposed to become. His probable death at Jaina's hands. His separation and alienation from his lover.
He really didn't need to know more. But it paid to listen a little longer, just in case he had to deal with them to protect his daughter.