In order to buy life insurance on another person you must have:
1. Insurable Interest in that person.
2. The person's consent/permission.
3. The person's signature on the application (usually).
And, the person you want to insure may have to answer some health questions, or take a physical examination to see if the person qualifies for life insurance.
Insurable interest basically means you rely on the person for financial support, or you would lose out financially if the person were to die.
Insurable interest does exist between spouses, siblings, relatives, parents and their children, business partners, etc.
Learn more about insurable interest and how it works.