I always find myself writing sentences that contain clauses within clauses, and I can never decide what the right way to punctuate this is. I'm not specifying what kinds of clauses because they could technically be several different kinds.
So here's an example:
I drove my little brother, Billy—the youngest of us, to the store.
This sentence contains two appositives—one inside the other. Firstly, I know appositives are usually set inside commas, but I feel like nesting them calls for using multiple kinds of punctuation marks. Secondly, is this even the correct way to punctuate this sentence? I feel like it could also be
I drove my little brother, Billy—the youngest of us—to the store.
This might not be correct because now the sentence is broken up by just one comma which creates two incomplete sentences. If so, why is it not correct to say the first example is broken up by the lone em dash?
Edit for clarification:
I'm pretty much asking if I can use the comma and em dash interchangeably here, and if so, which is better?