based on the release notes of the 2.02 mobile version is that all you need to do is share with her a Dropbox folder that starts with "YNAB".
From the 2.02 Release Notes:
It is now much easier to share a budget with another Dropbox account. Just share your "YNAB" folder with the other Dropbox account, and when you log in with that Dropbox account on another iPhone, the iPhone will be able to find all budgets inside of the "YNAB" folder, (or any folder that starts with "YNAB").
Sorry about the brother thing, I should have known better.