Query regarding account relationship

Scenario: The customer is a distributor. So the customer account is only linked to a bill to site and Does not have a ship to site. We ship the product directly to the final customer (distributor's client). The final customer only has a ship to-site. When i create a sales order for the distributor, I want to add the ship to site of the final customer.

Problem: It was my understanding that, to make this possible, I have to create a account relationship between both the accounts. But when I go to order creation, I can select ship to sites from accounts that do not have a account relationship with the customer as well. Moreover, if I had the account relationship for 'SHIP-to' enabled, I thought that the ship to address would be defaulted in sales order. But the ship to address is not defaulted at order entry