From a strictly logical point of view, even if there is a set of twins who are older than the third daughter, one of the twins will be older than the other. Not to mention that saying that one of them has blue eyes does not rule out the possibility that two, or even all three, have blue eyes. This means that even if Jet doesn't know which of the twins is older, if they both have blue eyes, then the older of them will.
However, if we assume that we have enough information* then there is a unique answer.
*You have a sphere. You drill a hole right through the center and measure the cylinder that the hole forms. How much volume is left in the sphere?
BTW: If you'd gone threaded, you could point directly to the post and not the whole thread, thus avoiding searching for the post as it becomes far up from the bottom.
http://wordsmith.org/board/showthreaded.pl?Cat=&Board=announcements&Number=131223
