You petition the court for guardianship. The court appoints a guardian ad litem, to evaluate whether guardianship is appropriate and who is the best person or entity to be the guardian. In order to attain guardianship, the court will need to find that your brother is incompetent (incapacitated). At that point, any contracts that he has entered into are void, because he lacks competence to enter into them. The debt collector would be out of luck.
Washington permits partial or full guardianship, so you or another party could be guardian of your brother's financial matters, but not over health care decisions; or any combination of involvement that is appropriate.
Anyone can petition the court for guardianship, although it can be a difficult without an experienced attorney. I do not specialize in guardianship, but would be happy to refer you to someone, if you decide to go this route. I would of course recommend seeking counsel. Feel free to email with additional questions.