Actual brother
It was her mother's command, after finding out Abrielle was the strongest witch born in centuries. Her mother tried every way to find her, even to the extent of telling her people that she was kidnapped by her father on the day of her birth. And who would not believe the Queen, she faked it like she loved her father even after her father's death that even she was fooled.
But like every lie, her mother's truth came to light too but it was too late. Abrielle mastered the art of magic and even became the beloved princess and hated her father all those years. Unlike they thought, Abrielle somewhere continued to hate humans and them being vulnerable. Because somewhere she refused to accept the truth, hoping that her parents really love each other and she was the proof of their love not just some unwanted child who killed her own father.