Question

In which sentence does the noun phrase "my oldest sibling" act as the subject

Answer 1

The answer is "Coincidentally, my oldest sibling, Nate, just turned nineteen."

Why? Well, if we look at A: "I" or the speaker in question is the subject talking about and or completing an action.

C: "My brother Nate" is already the subject, because Nate is the first person being spoken about.

Meanwhile in B: "Nate" Is not the subject, Nate is the name that describes, or specifies who the subject is.

So you got older sibling(Subject). Who is your older sibling? Nate,

Answer 2

The correct answer is - Coincidentally, my oldest sibling, Nate, just turned nineteen.
It is quite easy to spot a subject - just ask the question who. Who just turned nineteen? - My oldest sibling.
The first option is incorrect because the phrase is used as an indirect object. C is incorrect because my brother Nate is the subject, and my oldest sibling is the appositive. D is incorrect because Nate is the subject, and my oldest sibling is predicate nominative.