I've registered myself as a sole-proprietor business in Ontario, Canada.
My common-law partner often helps me with organization and other tasks. I want her to be legally able to purchase supplies with her own credit card, debit, or cash. And still be able to write them off as expenses.
Is this allowable by default or is there some sort of process for authorizing a family member to represent the business?
There is no process for explicitly authorizing this, besides legally classifying her as an employee or business partner. That said, it looks like you don't have to.
For tax purposes, your spouse is allowed to work for your sole proprietorship without being classified as an employee or as a business partner.Related: http://www.nolo.com/legal-encyclopedia/question-sole-proprietorship-spouse-28208.html