A (Not the best option, but it works:)
Define two fields in BrandANdSubBrand: BrandID, SubBrandID so that you get this relationship:
BrandANdSubBrand::BrandID = Inventory::fkBrand AND
BrandANdSubBrand::SubBrandID = INventory::fkSubBrand
I only posted this to help show why your current relationship doesn't work. Please use the following option:
B (A better approach)
Add __pk_BandSID as an auto-entered serial number field. Add a Brand and SubBrand fields in BrandANDSubBrand. Remove them from Inventory. Replace them with _fk_BandSID. Remove your BrandID field.
Define this relationship:
BrandANdSubBrand::__pk_BandSID = Inventory::_fk_BandSID
This pair of fields need not be displayed on any layout. They can be hidden from the users.
On any inventory layout where you need to show the brand and sub brand, add the Brand and SubBrand fields from BrandANdSubBrand to your inventory layout and they'll display the correct information. You can even sort on these fields or perform finds where you enter criteria into them.