Adelina Petrova was born out of wedlock. Adelina shared a mother with her siblings, but not a father, though only Adelina and her mother knew that. Her siblings father raised her as his own, as he thought she was. Adelina's true father was a warlock from another village. Adelina's mother thought Adelina was cursed because of her bastardness. Adelina couldn't preform magic like her siblings could, she could only perform if she held the hand of someone else during the spell. Adelina's "curse" was kept a secret until her family was murdered, and the murderer attempted to turn Adelina into a vampire. Her curse ended up being a gift, making Adelina the first ever vampire to be able to preform magic.
[TVD SEASON 2 - slow updates]