Without any input from the show, its very difficult to explain Candances origin.
We have confirmation from Lauren herself that Luna is Celestias only sibling. So it leaves us with few options
1) Shes Celestias daughter, due to alicorns being immortal, she could have had Candance at any time. Keeping their immortality in mind, its doubtful they age like other ponies, so even though she only appeared a teenager in twilights flashbacks, she could be alot older then that, as pointed out by several people in this thread already
2) All alicorns are related in some manner and she isnt directly related to Celestia or Luna at all and is a princess of another continent.
3) shes lunas daughter, but due to her being on the moon for 1000 years, this would place candance as being as old as the two sisters, which seems unlikely.