"Are you siblings?"
Translation:क्या तुम भाई-बहन हो?
You'll often see तुम लोग to address a group of people that are on the "तुम" level of respect with you (for example, classmates or friends) but it's kind of like saying "You folks" in English. The "folks" part is optional.
It's not like many European languages where the plural form is the same as the singular respectful form. If both the brother and sister are friends or otherwise on a casual level with you, it'd sound weird to suddenly refer to them with आप just because you were addressing them both at the same time.
It is indeed plural because we're addressing two people. The लोग is common, but optional, and more often than not is being used to nudge the language learner to construct a plural sentence. :) Both तुम and आप can be either singular or plural without need for a modifier.
Also what's been said by pilchat about तू is not correct; it is not a singular version of तुम. Don't use तू unless you know exactly what you're doing. In most contexts it'll be an insult.
There is a 'correct' order for such hyphenated words and inverting them makes it sound weird - भाई-बहन (siblings), दादा-दादी (grandparents), माता-पिता (parents) etc.
However, if you put in और in between like भाई और बहन (brother and sister), you can use whatever order you want though it becomes more specific.