Hi Caroline,
To the best of knowledge, Site will be treated as Customer in Inbound and Sales area distinction is used for Customers and not for articles. There is no way we can identify the sales area based on the brand of the article until you do an enhancement in POS Inbound.
In this enhancement, you can check the Brand of article and based on that value, choose the sales organization where you want to post the billing document.
Regards,
Amit