"Do you have a younger sister?"
Translation:У вас есть младшая сестра?
There might be a better (grammatical) explanation for this, but in general есть is necessary in "Do you have ..." sentences. You can omit it only when you want to ask about the adjective (Is your ...). 'Younger' is not the best example to illustrate the difference (both in Russian and English), so I'll give examples with another adjective:
- У тебя есть большая квартира? - Do you have a large apartment?
- У тебя большая квартира? - Is your apartment large?
- У тебя квартира большая? - Is your apartment large? (also correct, with English-like word order).
I was taught by my Russian professor that есть was never necessary any time there was an adjective. I do understand your explanation and I can see how it changes the context of the sentence, but I'm still surprised Duolingo insists есть is necessary.