Yes, adults may purchase burial insurance for their brother or sister.
In order to purchase burial insurance on someone else you need insurable interest on that person and the person's permission, usually - however, that depends on the insurance carrier.
Insurable interest means you either rely on the person for financial support of some kind, or that you may suffer a financial loss of the person were to die.
For example, if you loaned your sister money and she died before it was paid off, you would lose that money you loaned her.
Or, if you had to pay for your brother's burial because he didn't have enough money saved to cover the full cost of his burial expenses.
Learn more about buying burial insurance for your siblings.