We do a lot of business with international companies that have various branches across the globe. It would be useful to be able to associate these branches with each other through a "sister" or "related" company relationship, similar to the parent/child relationship we currently have.
Likewise, it would be useful to be able to add in more than one company domain.
Some companies treat their branches as seperate, but equal entitities to themselves (which is where a sister relationship would need to be used), while others treat them as an extension (one company with multiple domains, due to existing in multiple countries).