Is a Ford a Mercury?
No, but Mercury's are Fords. The sub-brand is a product of the parent company. Also, for the past 20+ years they are virtually indistinguishable from Ford brand products.
Is a Chevrolet truck a GMC?
Yes, they are virtually identical. Different plastic bits and different dealers may change the way you think about them, but they are the same
thing.
Is a Honda an Acura?
No, but Acura's are Hondas. In fact, in some markets, this is literally true not only because of the parent company issue, but due to cross-marketing. Heck, in many markets the NSX was sold as a Honda, and some of the other current US Acura offerings are also sold as Hondas overseas.
Is a Toyota a Lexus?
No, but Lexus's are Toyotas. The sub-brand is a product of the parent company.