When it comes to buying life insurance on the life of another person, there must exits an insurable interest between you and the person you want to insure.
Insurable interest means you have a financial interest in another person, or you may stand to lose financially if the other person were to die.
An insurable interest may exist between spouses, parents and children, siblings, relatives, and business partners.
However, when you apply for life insurance on your brother, you may need his permission, his signature on the application for life insurance, and he may need to answer some health questions, and/or take a physical exam to qualify for the life insurance policy.
There are some life insurance plans available online that do not require a physical exam.
Here's how you can learn more about What is Insurable Interest and how it works.