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Is there any "logic" behind such an unusual construct ( = IS is followed by a noun in the plural)?

Perhaps there are other English expressions/constructs following the same pattern, i.e. "Person A is <noun in the plural> with Person B"? e.g. "Nick is enemies with Bob" or something :-)

Comments  

The given sentence is okay. This is an idiomatic-type construction and therefore does not need to be strictly grammatical. The meaning is (here's where a kind of logic behind the construction can be seen): Sally and Tom are friends. Other constructions similar to this are, for example:


Nick is enemies with Bob. (= Nick and Bob are enemies)

I'm enemies with John. (= John and I are enemies)

I'm friends with Jim. (= Jim and I are friends)

He's lovers with Sarah. (= he and Sarah are lovers)

He's co-workers with Dave. (= he and Dave are co-workers)

He's classmates with Jane. (= he and Jane are classmates)

He's teammates with Bill. (= he and Bill are teammates)

He's relatives with Jan. (= he and Jan are relatives)

Absolutely unexpected outcome...
My special thanks for giving me simple yet very useful examples! ( I wish I could put a bunch of likes at a time on your reply!)