I posted this in a separate thread yesterday but I believe that Sylvanas is going to finally see her sister Vereesa Windrunner during the Siege of Orgrimmar. Vereesa was with Jaina for the entirety of the Isle of Thunder, so it's plausible to believe that she will also be Jaina's lieutenent during the Siege. This makes things very interesting because Jaina and Lor'themar interact, and it's clear that Sylvanas is with Lor'themar during the Siege. So there is a small chance that Vereesa and Sylvanas might come face-to-face, which might explain Lor'themar's quote in his sound files where he asks about 'what has come over our Dark Lady?'