If you are buying life insurance for your grandmother to cover her funeral, burial, cremation, memorial service, cost of flying guests in for the funeral, etc. Yes, for reasons of final expenses, you can buy life insurance on your grandmother.

If you are living with her and would be financially impacted if she were to pass, you can buy a life insurance policy on your grandmother.

These are examples of when you can get life insurance on your grandmother. 

However, in order to buy life insurance on your grandmother you will need her 

consent/permission, signature on the application, and she may need to answer some health questions or take a medical exam for the life insurance coverage.