Most likely not, based off of the fact that Twilight is only Celestia's student, and never considered anything more. Since Luna is Celestia's sister that would mean since Twilight isn't shown to be related to Celestia, she can't be related to Luna. She is however related to Shining Armour. He is her brother.
No, Nightmare Moon is not Twilight Sparkle's daughter. Nightmare Moon is actually Princess Luna, Twilight's fellow princess and sister. She became Nightmare Moon after being consumed by jealousy and bitterness, but was eventually restored to her true form through the efforts of Twilight and her friends.
the elements were destroyed remember! this means that she had to recreate them using twilight sparkle an all her friends its the friendship that is the elements also its kindof for entertainment purposes too!
Princess Celestia, the Great and Powerful Trixie, Derpy Hooves, Vinyl Scratch, Rarity, Applejack, Twilight Sparkle, Pinkie Pie, Fluttershy, Rainbow Dash, Princess Luna - actually everypony's best pony!
No. Looking at the flashbacks to when Twilight was a filly, Cadance is noticeably younger. This suggests that she is relatively young, meaning that whe would have been born some time during Luna's 1000 year banishment as Nightmare Moon. The description of Cadance as Celestia's niece is not meant in the literal sense of the daughter of a sibling. If you believe the story in the book Twilight Sparkle and the Crystal Heart Spell, then Cadance is not even related to Celestia by blood, but was essentially adopted.
there is no princess in Costa Luna because it Costa Luna is just a make up place.
No. The focus of My Little Pony: Friendship is Magic, is on Twilight Sparkle, Princess Celestia's student, and the show so far was leading up to how she became so educated in the magical arts that she became one of four alicorns in Equestria. Keep in mind that it is not the same case for the other of the Mane 6, and they have no reason to become alicorns.
Costa Luna
No
Twilight Sparkle is one of the main characters on the "My Little Pony: Friendship is Magic" television show. She is Princess Celestia's faithful student and is studying the magic of friendship in the small town of Ponyville.
So far there has been no mention of how Luna and Celestia came to be.
Yes, Princess Celestia, Luna, and Cadence will get their magic back since the main 6 defeated Tirek and have been rainbowfied to help Equestria receive their magic back. If you want to watch it you can on Youtube or Dailymotion
Princess Luna became jealous of Princess Celestia and tried to take full control of Equestria and bring everlasting night. Celestia was forced to banish her to prevent this.