In order for someone to buy life insurance on another person you may be required by the insurance company to have an insurable interest in the person you want to insure.
This means you rely on the person or persons for some form of financial support, or it would cost you financially if they were to die.
In addition, family members, spouses, siblings, parents and their children, and business partners, all have an insurable interest in each other.
So, you may buy burial insurance plans on your grandparents, as it can help to pay for their final expenses after they have passed away.
Here's an article that explains can you buy burial insurance on someone else.