The other day, my father and I were expecting my brothers to come home. Upon hearing a car enter the driveway, my father said, "Your brothers are here."
When I looked at the door, I could see that it was actually my sister who had arrived, and not my brothers, so I responded,
She's not my brothers.
This seemed like a very awkward construction, but I could not think of any reason that it would be ungrammatical, other than the subject being singular and the object being plural.
I can think of several examples of sentences where the subject and object are different in number (with an intransitive verb: obviously there is nothing wrong with a transitive verb like "kick" having a subject an object differing in number) that do not seem to have any grammatical awkwardness.
We are a family.
His mind and body are one.
God is three persons: the Father, the Son and the Holy Ghost.
Is the sentence, "she's not my brothers" grammatically correct, and what makes it different from these other sentences?