Anne, in the long run, if you or others keep doing her work and she passes and becomes a nurse, how many will be hurt from her lack of knowledge?

I think you have done what you can.

If her standard of doing well is a 60%, like Alffe said she needs to grow up and accept responsibilty.

If her family doesn't have the money, then she needs to apply for a work permit ( or what they have there down under) and earn her own way to take these course again and pass at a higher %.

Find all the resources you can. Give them to your cousin. Not to be harsh, but it sounds like she passing this all onto you. When you take in an exchange student, they become your family while they are there.

You are not to blame, no matter what.