Well, in order to purchase life insurance coverage on another person, you need an insurable interest in that person.
This means, you either rely on the person for some means of financial support, or that you would suffer some sort of financial loss should your sibling die.
For instance, does your sibling support you financially, owe you money, or would you pay for you're sibling's burial costs upon his.her death?
If so, then you have an insurable interest in your sibling, and you may buy life insurance on your sibling.
When purchasing the term life policies on your siblings, the insurance company may require that your siblings sign the application for life insurance.
Usually, you need permission from the person you wish to insure.
Learn more about who you can insure for life insurance and how to compare term life insurance rates and plans online.