I respectfully disagree though I should have capitalised less carelessly - 'Sister studies' feels to me fine without an article - compare: 'Father arrives', 'Mother disapproves'.
TBH to my ear, the sentence 'The sister studies' is itself rather awkward and contrived, and perhaps this is the issue. 'Sister' really needs some clarification of the relationship (presumably a pronoun rather than the article) - 'my sister studies', 'her sister studies' are much more plausible sentences. 'The sister studies' feels like a fragment of a sentence at best - in a lifetime, have you ever said this sentence?
So all in all, I still think 'Sister studies' is ok - because it reads implicitly as '(my) sister studies' or (eg of a nurse or nun) 'Sister (Mary) studies.
"Father" and "Mother" are often used as titles/names (like "Mommy/Mummy", "Daddy", etc.), which don't require articles, but I don't hear "sister" and "brother" used the same way (unless referring to a nurse/nun/monk, when I've only encountered it accompanied by a name unless directly addressing the relevant sister/brother).
I agree that "The sister studies" is kind of awkward, but it's difficult to come up with an English translation of the Latin sentence that doesn't, without adding in something like a possessive which isn't in the Latin.
Hospitals and convents are perhaps the only places you'd hear eg 'Sister is waiting for you', so I agree, but really I suspect all this is clunky maybe because it's something of a clunkily-chosen example phrase? "Livia studet" eg might have been clearer, and then get into family members only after introducing the possessives that make them clearer?
That maybe would have been better, but I assume the contributors probably had a reason for it. I'm not one of the regular contributors, just temporary help with the initial flood of reports, so I wasn't involved in the decision making process. :) At any rate, we're stuck with it until a new tree version is made now, which means we're also stuck with this kind of clunky sentence as at this point we don't have the vocabulary available to make anything better.
Father and mother used as titles:
Bother and sister used as title can be ambiguous, because they are religious titles. But "father" and "mother" too.
So I don't understand why it's possible in one case, and not in the other case. I would need rules.
I thought I'd share this nugget since Sicilian is my first language (the first language I was raised with & many believe it's the closest language to Latin).
"Soror studet" sounds like the Sicilian spoken in my parents town. BUT the last "r" in "soror" is soft/almost silent, and the last "t" in "studet" is also soft/almost silent.
So saying this sentence "Soror studet" would literally mean "(My) sister is studying" or "(My) sister studies", depending on context.
They accept now "My sister studies".
That's true that possessives are often implied in Latin.
But in this kind of sentence, you would add an additional meaning, as you have no way to know if you talk about your own sister or someone's else sister. The possessive is not obvious at all, so it's not a good translation I think. In other sentences provided by Duolingo, sometimes, it's really more obvious that a possessive could be used.
For instance, in sentences like:
I.... the sister (in Latin)-> my sister (English)
You.... the sister -> you sister.
With this kind of sentence, I think that the implied use of the possessives in Latin are more obvious and natural, than in a short sentence with no pronoun as an additional information.
Why can't "Soror studet." be "Your sister studies"?, as you address someone, why it'll be more "my sister"? (I mean in the Latin sentence, NOT in the English use)
Coud it be translated by "The cousin studies".
I know that "soror" is sometimes translated with "cousin", but I don't know if there's a specific context.
In the Bible, they translated "Jesus' cousins" with "Jesus' brothers", so I think there's no context, and that "cousins" could be accepted.