Reen raised Vin after finding their youngest sister dead and their mother insane. Vin's mother had pierced Vin's ear with a bronze earring and was calling her a queen. He took Vin and fled.
Vin and Reen avoided attention by living in the skaa underworld, rarely spending more than a year in one place. They traveled the waterways with the skaa canal crews and lived with thieving crews. Reen beat Vin if she wasn't quiet and unobtrusive, or if she did anything to jeopardize their survival. He taught her to be suspicious—even paranoid—and to do whatever was necessary to survive. Because of this treatment, she became emotionally frail and had an astoundingly low sense of self-worth.
Reen told Vin repeatedly that no one could be trusted and that everyone would leave and betray you if given the chance, even telling her that one day he would leave her.
Reen made good on his word, disappearing when Vin was around sixteen years old, leaving her with Camon and his crew. Her wages were docked and she was beaten frequently because of her brother's debt. Only later did Vin learn that Reen had not left: he had been captured by a Steel Inquisitor and died insisting she had starved to death years earlier, even under torture.
Over the years Vin would hear Reen's voice in the back of her head, always suspicious, cynical, and distrustful. While trapped in the Lord Ruler's underground cache in Fadrex City, Ruin manifested as an image of Reen, attempting to manipulate Vin through her memories of her brother. Vin saw through the ruse, causing Ruin to mockingly reveal that the voice in her head, rather than being memories of her brothers advice, was often Ruin, attempting to manipulate her into serving it.
Reen's behavior towards Vin was brutal. However, because of it Vin learned many of the skills she needed to be a survivor. Only after a Steel Inquisitor revealed Reen's fate did Vin realize the full truth about her brother. Reen was not a good person, but he treated her as he did because he feared for their safety. His attempts to keep her hidden and unnoticed were a desperate effort to escape detection by the Steel Ministry, especially Inquisitors, or by anyone who may figure out she was half noble and turn them in.
In the end, Vin realized Reen loved her.
- ↑ http://www.brandonsanderson.com/annotation/108/Mistborn-Chapter-One-First-Half
- ↑ Mistborn Chapter Thirty-Seven Part One Annotations by Brandon Sanderson, http://www.brandonsanderson.com/annotation/170/Mistborn-Chapter-Thirty-Seven-Part-One
- ↑ The Hero of Ages, chapter 44