If her brother was her guardian, then the marriage is valid and they have no right to take your wife away from you. This is wrongdoing on their part. They had no right to harass you in order to take money from you. The wealth of a Muslim is not permissible to anyone unless he gives it willingly.
You have to go to the sharee’ah courts and turn to the scholars and prominent figures in order to resolve this problem.
With regard to killing him and killing any member of his family, all of that, as you mention in your question, is a major sin that is worse than blowing up the Ka’bah.
Whoever does that, the curse of Allaah, the angels and all the people will be upon him, and his recompense will be Hell, to abide therein for eternity.
We ask Allaah to keep us safe and sound, and to set the affairs of the Muslims straight.