Discipline is the job of the parent and should never be passed down to the younger siblings. It is not fair, and it does not allow the siblings to have a natural relationship. When a younger sibling is being disobedient and not listening to the older sibling, the older sibling should simply go to the parent.
Studies have found that when most individuals are given the power to punish their piers they abuse that power. Children are not mature enough to handle having power over another person and still act responsibly. Few children know what is an acceptable punishment as well or the effects their punishment can have.
Parents should be the ones who deal the punishments. Siblings don't particularly understand well the concept of punishment. Leave this job to the parent. They will make a much better job of it than the sibling, as they will most likely understand the problem better than the "older" sibling, however old that sibling may be.