[Antoninus]

The greatest gift ARE siblings? You have a singular subject and a plural "object" connected by "be". You can avoid the problem by pluralizing "gifts" but let's suppose you want to keep it singular.

When one has a linking verb, is there a rule for which word is the subject and which is the predicate nominative? If so, I've never heard it. I just pluralized everything. Hopefully, that will satisfy the grammar gestapo. :-)

[Claud]

I think the one after the verb is the predicate nominative.