Number agreement between subject and object

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?

Solution 1:

Assuming said with emphasis on she, I think "She's not my brothers" is natural enough, and it's not clear what you'd say instead. Possibly it just sounds slightly awkward because there aren't so many contexts where it is pragmatically valid to talk about the equation or non-equation of one person to two people.

But grammatically, the sentence is just following the usual pattern. Because English is strongly Subject-Verb-Object, with "be", the element before the verb is usually treated as the subject and so this is what the verb agrees with.

Solution 2:

She's not my brothers.

is grammatically correct if you put quotes around 'my brothers', as I have just done. This allows 'my brothers' to be treated as a singular object even though there are more than one 'brother'. "She is not 'my brothers'."

You run into the same awkwardness when explaining that you are looking for your scissors but have only found your pencil instead. "This pencil is not my scissors."

Solution 3:

Whilst maybe grammatically correct it does sound awkward, I would agree with Neil Coffey here in that it is probably awkward due to the issue with one person / two people.

The three examples you provided are slightly different:

We are a family.

Here, we is a group and family is a group therefore there isn't the same conflict.

His mind and body are one.

His implies ownership; he can own more than one thing.

God is three persons: the Father, the Son and the Holy Ghost.

It is clearly stated that God is three persons. People with multiple roles can be referred to in a similar way; someone can be both a father and son without there being any form of conflict.

Although She's not my brothers is grammatically correct, a probably-better way to respond is "No, it's my sister".