I also heard that Cadence WILL be outliving Shining armor.



now aside form that; knowing Celestia as a bit of a trickster mentor she might just say "Time will tell"...although I don't see her as one to take unnecessary mistakes [she CHARGED at a Changeling queen moment her cover was down...granted that wasn't the BEST choice right there but she was counting {most likely} on her being stronger than the queen in terms of magical energy; but due to feeding off of shining's love for cadence for most likely three or four days she was more powerful than Celestia figured] back on track; true she may try to do a trickster thing but considering twilight's sanity slippage in S2 she won't want that to happen again with her worrying several times over; so Twilight probably knows [which is why she's calm and cool and the only things shaking up her sanity is the occasional Pinkie Pie or Discord visit] but we don't get to know due to keeping us in suspense.



Now should the Mane six become Alicorns alongside Twilight and if Cadence will be outliving Shinning Armor, and if Twilight remains immortal; then doesn't that mean that Twilight will outlive her brother?



Now lets also take into effect of the Mane six becoming immortal Alicorns; Fluttershy, as it stands, is a mortal pony, Discord is not, moment Fluttershy passes due to old age, what will Discord do? Will he curl into a ball, hide in a cave and weep his sorrows away and then come back thousands of years later feeling better? Or will his mind break in his sorrow and we have a repeat of the Discopian era? [I refer to the time-period in MLP cannon of Discord's rule as "Discopian"] So, if Celestia is indeed not taking any unnecessary risks she will see to it that Fluttershy [at least] becomes an immortal Alicorn as then Discord breaking or not will not be a question on Celestia's mind [a worried ruler is an unfit ruler] and once again, we are being kept in the dark for the sake of suspense.