If we're going to be prescriptivist about things and not accept colloquial English, "Whom is your sister with?" would not be correct either. Grammar aside, I am almost certain that most English speakers would phrase this as "who is your sister with?"
Yes, both of these "correct" answers are incorrect. "Who is your sister with" or "Who is with your sister" would be how I would normally phrase it. I reported the problem(s).
Technically, under strict English rules of grammar, the "correct" answer would be "With whom is your sister?", but that is so atrociously awkward, nobody would say it that way.
The problem with the colloquial correct answer is that the phrase "Who is" sounds like a subject-verb pair, which is an explanation of why it is used so frequently. Of course, it's not a subject-verb sequence - but it sounds so much like one that many English-speakers are more comfortable with it than "Whom is", which sounds weird.
1) С КЕМ твоя сестра - your sister somebody with anywhere ( with parents or friends etc)
2) Твоя сестра с КЕМ it is need understanding as "who is your sister's boyfriend"
it is not strong rule but such is the context
I tried the horrible phrase "Whom is with your sister" because I thought that is the sort of horrible phrase DL wants but got it wrong anyway. I couldn't remember the horrible phrase DL preferred! It is very difficult for me to remember mistakes correctly.
you can't say "whom is" since "whom" is only supposed to be used as the subject of the sentence. Think he=who and him=whom
"him is with your sister" makes no sense, but "he is with your sister"/"your sister is with him" does. Similarly "Whom is with your sister" doesn't make sense, but "who is with your sister"/"your sister is with whom"? does
Yes, I thought this, too. I went against my better judgment and ended up getting it wrong.
"Who(m) is your sister with" is technically not proper grammar, but it's how 99.9+% of English speakers would say it, even those who know it's "wrong". I think accepting it is as a translation is best -- there would be lots of complaining if it wasn't. I wonder if "With whom is your sister?" is accepted...
"Who is your sister with" is proper grammar. This isn't the first century.
Also, if 99.9+% of speakers say something a certain way, then that way is proper.
"With whom is your sister?" is accepted (April 28, 2016). :-) Though I think it should really be the default answer (with the others as alternatives, of course). Since the Russian pronoun is not in the nominative case, the appropriate English pronoun is the object pronoun.
If I say "Who's with your sister?", would it mean differently with the one in the suggested translation?
No, no difference in meaning at all.
That said - I think for a while, not sure if it's fixed, they weren't accepting "Who is with your sister" (which is essentially what you wrote).
I'd report it if you got it wrong and you run into it again.
Thanks, yeah I wrote "Who is with your sister" but it wasn't accepted so I thought maybe they have different meanings. Thanks for clarification :)
an irrelevant question: "who was your sister with?" would be "С кем была твоя сестра?" correct?