I'm not sure I understand why you say the verb is in the second position in Meine Schwester wohnt in Berlin. Isn't it in the third position?
Ah, we don't mean literally the second word. Meine Schwester here counts as one thing, or fills one position. It is the subject of the sentence. German is a subject-verb-object (SVO) language, same as English.
Also, making questions in German is much easier than in English, isn't it? No extra helping words needed and the verb is at the very front!
Yeah, I suppose that's easy enough. By the way, what's a "main clause"?
For now, it is enough to say "main clauses" are normal clauses. When we learn "subordinate clauses", we will go deeper into that.