The sister is in the city, but the brother is at home.

[Kelikaku]

[McGonnagle]

In my opinion, Town (oppidum) is smaller than City (urbs).
Thus, the Latin translation for your English sentence would be,

Soror in oppidō est, sed frāter domī est.

[McGonnagle]

Thanks! Yes, I've heard native English speakers speak like this, but I didn't know what exactly it means. (I was only guessing it from the context.) It's great I can learn English here, too! :)

Duo accepts answers without "the" in most of the cases.

To learners, according to Latin dictionary, urbs is a city or a walled town, while oppidum is town (esp. of towns other than Rome, which was generally called Urbs)

So Kelikaku's answer is not technically wrong, then! Probably just like superessedi said, DuoLingo drills usually ask for a quite literal word to word translation.

[Kelikaku]

... I gather that “to be in town” is an idiom used with cities ...

I guess that is true. Even if you were in New York City or Washington D.C., you would refer to this as "being in town."

Noone would ever say, "I'm in the city"; one would state that "I am in town." Maybe if it were an invasion, and a scouting expedition were reporting to their HQ. "We have just breached the outskirts of the city; I am in the city now."

Saying "in the city" connotes an unwelcome entry, saying "in town" connotes a mundane or social visit - even in a business context, "in town" is much more of a standard expression.

[McGonnagle]

... I gather that “to be in town” is an idiom used with cities ...

I guess that is true. Even if you were in New York City or Washington D.C., you would refer to this as "being in town."

Noone would ever say, "I'm in the city"; one would state that "I am in town." Maybe if it were an invasion, and a scouting expedition were reporting to their HQ. "We have just breached the outskirts of the city; I am in the city now."

Saying "in the city" connotes an unwelcome entry, saying "in town" connotes a mundane or social visit - even in a business context, "in town" is much more of a standard expression.

English isn't even my second language, so I'm not in a position of being able to comment on this, but I think it depends on where the speaker of this sentence is. Or maybe the background of this sentence is the ancient Roman Empire, and urbs at this time meant the area "inside of the wall of Rome", so they say "in the city".　　

Apart from that, the word list of my two Latin books (Lingua Latina-a comparinon to Familia Romana and Oxford Latin course, part II) just simply says "urbs -bis f city". So doing a Latin beginner's drill, the answer by literal traslation is more likely accepted, probably it helps to avoid a confusion of the meanings between "urbs (city)" and "oppidum (town)" at least in my opinion. The main point here is learning Latin words.

If you don't like the English sentence, report to Duolingo, they may help you to change that.

Happy learning!

[Kelikaku]

... it’s a valid translation in English, you can report it ...

There is a similar concept in Hebrew and in the Talmud. A "walled city" is simply a city, but if it is a settlement without walls, it is not technically, in the legal sense, a "city."

[angsthasen]

To be in town has a slightly different meaning that to be in a town.

If I were to say, "My sister is in town this week", people would understand me to mean that she lives elsewhere but is currently visiting my town, the town where I currently am. I would usually use this phrase when excusing myself from social activities with somebody else, since my sister is in town and I have the rare opportunity to visit with her.

On the other hand, if my sister has a home in an urban area and one in a rural area, it might be appropriate to say, "My sister is in the city, this week" to express that she is not at her home in the countryside.

Given how often we've been told "Soror in urbe est; frater domi est", my mental model is that the family lives outside the city, but the sister sometimes travels there for work or entertainment.

My personal experience is of living an hour or two from the major city of Toronto. If I wanted to watch live theatre, I might go into Toronto for the show. At such time, if somebody asked, "Ubi est Angsthasen?" it would be appropriate to say of me, "Angsthasen in urbe est." If I were meeting a friend who lived in Toronto, they might say, "Angsthasen is in town," but it would still be an unlikely answer to "Where is Angsthasen?"

[Kelikaku]

... to say, "My sister is in town this week", people would understand me ... it might be appropriate to say, "My sister is in the city, this week" ...

Well, if you lived in a small village and were on your way to the closest city, you would say "I am going to town" but you most likely wouldn't say "I am going to (the) city."

There are some people who I have heard who do speak that way though. I understand that people who live in New Jersey or Long Island, NY will actually refer to themselves going "to the city." In my opinion, those are a minority of people. I know that there are other urban areas which people will refer to the major urban zone as "the city." I think that London, England is also another example. These cases, I believe, are limited to internationally recognized and traveled-to towns, like New York City; London, England; San Francisco, California; Seattle, Washington (even people who live in Seattle, I know will refer to the central area as "the city," despite the fact that they live in the same municipal area as "the city"). But most major urban areas, I do not believe it is that common, to the point of being a little strange. Most of the time, people in the USA would say, they're going "to town" or they're going "downtown."

[angsthasen]

Well, if you lived in a small village and were on your way to the closest city, you would say "I am going to town" but you most likely wouldn't say "I am going to (the) city."

No, I really would not say that.

"to go to town" is an idiom meaning "to do something thoroughly, enthusiastically, or extravagantly."

I might say I'm going into the town, but going to town has a separate meaning.

I understand that people who live in New Jersey or Long Island, NY will actually refer to themselves going "to the city." In my opinion, those are a minority of people. I know that there are other urban areas which people will refer to the major urban zone as "the city." I think that London, England is also another example.

This is actually the meaning of urbs. Towns and small cities were not urbs. "Urbs" alone could be understood to refer specifically to Rome, the megacity of the time. We see this in quotations like, "Ab urbe condita" and "Urbi et orbi."

If you want a city that isn't a centre of world commerce, consider oppidum. For smaller settlements, perhaps oppidulum, vicus, or rus would serve you.