They could have made this sentence read "Ustedes son los primeros" and it would have been right. This would have been referring to an all male group or a mixed male and female group. They chose instead to make the sentence ".....las primeras" which refers to an all-female group. Both are right. For practice they just choose to use one sentence over the other. The important thing in a real situation is to know when to use each sentence.
This is now accepted as a correct answer. Just as we often need to add "It" to make a sentence work in English, sometimes we need to add "one(s)" -- or at least to make it more natural sounding in English. In this sentence since the "you" in question is plural, "you're the first" just didn't sound clear enough for me, so I specified "You're the first ones" as in "You're the first ones to arrive."
context, context, context! And when there isn't any, keep it simple. We don't really know what they are talking about, so best not to add our assumptions. so many times I have seen someone post that no one would ever say ____ so it can't possibly be a correct translation (and in my mind sometimes I agree), but then someone posts a scenario in which it's totally plausible. the moral of the story: stick to the facts. not picking on you specifically ;)
I don't think I would ever say "you're the first ones." I would probably say "you-all" are the first" (knowing full-well that "you-all" is not standard English.
Or I might say, "this (your) group is the first."
I also might likely say "you're the first."
By the say, "y'all" is a southern (U.S.) expression. It is pretty much never said in the other 33 or so states except by southerners who happen to be living/visiting in those other states. It also might sound phony or as an affectation, if it is not natural to you.
Y'all is definitely not standard English. You'll never find newscasters saying it, except perhaps by a few, if any, southern newscasters.
I wonder why DuoLingo never mentiones "vosotros" and even "ustedes" barely comes up... I learned about vosotros in another website, and I was frustrated why Duo never mentioned it once up to this point. Is it an obsolete use or something? If so, why is ustedes almost never mentioned as well?
"Vosotros" is primarily used in Spain to address a group of people directly with whom you are familiar; most Central and Latin American countries do not use vosotros and use "ustedes" in all forms of plural address (spanishdict.com). Duolingo uses this Central/Latin American Spanish. I'm not so sure about Ustedes almost never being mentioned as you say because I've had many sentences with them here on Duo. Maybe you'll get to meet more of them here soon.