A trailer revealing the twins has been released.
BOOM video 6516
Numerous balance tweaks have been documented by the fighting game community from playtime at arcade location tests in Japan. Though not official, the list of suspected changes can be found over at Shoryuken.com
Sadly, a PC port has not yet been announced. It is also unknown if Yun, Yang, or these balance tweaks will be making their way to the Xbox 360 and PlayStation 3 versions of the game.