Honestly, people. She just has wings now. Hasbro knows about the immortality issue in the fanbase and kept Twilight mortal, like Cadence; they said so. If they were supposed to be immortal alicorns, they would look more like Celestia than normal ponies. So keep your pants on.
I know; the original script of the first episodes included Celestia and Luna's parents, said to be deities of Equestria, and the princesses inherited their immortality from them...but they removed them for story purposes.
I know that for a fact; it's only been one week since Twilight's ascension and I've already lost count of how many immortality-themed stories I've found on Fimfiction, and don't get me started about all the drawings; they have a deep, emotional touch to them, but it just makes me worry.
But it is nice to know; no need to worry Twilight about outliving her friends anymore...though that still leaves us with the problem of Spike outliving them, being a dragon and all. Hasbro has yet to announce that resolution.