Learn German: Seid ihr Geschwister?

Seid is the ihr (plural informal "you") form of the verb sein (to be). Because this is a yes/no question, the verb moves to the very beginning of the sentence.

Why use "ihr" instead of "Sie" or "du" for "you"?

ihr is the informal plural "you" (like "you guys" or "y'all"). Since you are asking multiple people if they are siblings, du (singular) doesn't work. And because you are asking about a close family relationship, the informal ihr is much more natural than the formal Sie.

Why isn't there an article like "die" (the) before "Geschwister"?

Geschwister means "siblings" and is a plural noun. Just like in English ("Are you siblings?"), you drop the article here because you are asking about their general category or relationship, rather than pointing out a specific group of people.