"How is your sister today?"
Translation:Come sta vostra sorella oggi?
Hi Thomas. I think, as in Portuguese, there's a differentiation between two significances in the verb to be: 1. To be as a temporary state: "she is happy today"; 2. To be as a permanent state: "she is tall". In this case, the state of well being, I believe is better to use STA instead of È.
There are already several threads on this very topic, so I won't go too much into detail, but the short answer is the following: for family members in their singular form, one doesn't usually use the article before mio/mia. E.g.: "mio padre" and not "il mio padre", "mia sorella" and not "la mia sorella". However, this only works for singular nouns: "La mia sorella" is wrong but "le mie sorelle" is correct.
They're all equivalent... "Come va?", "Come stai?", "Com'è?" (north Italy slang)... BUT those three are used when referring to someone directly (how are YOU)... You CAN'T say 'come va tua sorella' or 'com'è tua sorella'... These questions would have other meanings (the first would literally mean how is your sister going, like by car, by bike, whereas the second would mean how is your sister like)
There are so many nuances in a language. If we understood correct Italian, we wouldn't be here studying! ;-)
I think how we come to know it is just going over the sentences until the structure starts to feel natural to us.
I know it's frustrating to get your answer marked wrong when you think it's right, but I'm deferring to the experts. Sometimes they're not consistent, but living language is like that. This is a hell of a deal, and I'm beyond grateful that this site is free. I can practice all I want until I get comfortable with it.
"Vostra sorella" is just used because you are talking to more than one person. It would be used if someone was facing my brothers when I'm not around and they asked my brothers "how is your sister today?" Perfectly reasonable sentence.
If they said "Come sta tua sorella?" when facing both of my brothers, the question asker would be deliberately directing the question at one brother and ignoring my other brother which would be pretty rude.
If you are facing a group of people who don't share a common sister - ie two friends - then, yes it is nonsensical. You would probably say "Come sta le vostre sorelle?" instead.