My wife and I use consequence to emphasize that our children are responsible for their actions and thus also responsible for any punishments they receive as a result of misbehavior. We impose punishments sparingly, and when a punishment is called for, we also hold a discussion about why the child's actions warranted punishment. Consequence helps make that link.

In the case of this letter, I agree that a lock is the best solution, coupled with a discussion with younger sibling about why older sibling deserves a lock to protect their privacy.