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 elder sister realized she was with child, and in a way to promise her sister that she would keep the secret, she shared her own.
[TVD SEASON 2 - on hold]