Translation:This boy likes to watch TV with his brother and with his sister.
I'm confused: in English it would be either "with his brother and sister" or "with his brother and his sister." We would never say , "with his brother and with sister," but this is the word-for-word translation. Repeating the "with" would remove the connection between "his" and "sister" in English. But apparently not in Russian! Is this typical in Russian? How do we know it is "his" sister when this doesn't seem to be specified?
With "с братом и сестрой" you omitted both "своим" and the second "с." This is easy for me to remember because is is pretty close to a word-for-word translation from English. The second "с" was bothering me and I'm glad to know that it is optional, apparently redundant and not entirely necessary.
In English, the second "with" is clearly redundant, though not grammatically incorrect. And it sounds like the Russian situation may be similar.
The preposition is usually not repeated for the sake of the euphony but that's all. Don't forget Duolingo is an educational resource. Students translate their phrases word-for-word. This explains why we get a little bit weird sentences. Nevertheless, this sentence sounds good to my ear.
"Со" is used when the next word starts with a consonant cluster containing one of these: л, н, р, с (e.g. "со льдом", "со снегом", "со всеми"). When the next word starts with an "с" followed by a vowel, using "с" instead of "со" sounds more natural.
On a side note, you can also use "со" when it's followed by a word starting with a "щ". Although in modern spoken Russian it's not uncommon to use "с" in these cases as well.
I hope a native speaker will answer, but I will try. The nominative singulars are дверь and сестра. One ends in a soft-sign and the other ends in a hard vowel. The other endings in the declensions follow suit. "Ю" is also soft. "Ой" is a hard ending, although Wiktionary shows an alternate instrumental ending of "ою" which satisfies the requirement for a hard vowel but adds the "ю' that you are wishing for.
No, you're absolutely right. Given the right context, "этот" can translate to any of those things, as one of its meanings covers things/people that have been mentioned last. So if this sentence was part of a more elaborate exchange, 'that boy' could even be the preferred translation.
E.g. 'Remember I told you about Helen's fidgety newborn? Turns out, that boy loves watching the telly with his brother and sister.'
"Помнишь я тебе рассказывал про беспокойного нового отпрыска Хелен? Оказывается, этот мальчик любит смотреть телевизор со своим братом и с сестрой".
On its own, however, this sentence is less likely to refer to an already mentioned boy, but rather points to a boy in the speaker's physical proximity.
In Engllish "this" and "that" refer to the degree of proximity : "this" for something close at hand, "that" for something farther away, whether in space or time. Does Russian have this difference? (I often use "that" in translations assuming that the speaker wouldn't talk about another person, especially a child, in their presence.)
Yes, the main difference between the Russian pronouns "этот" and "тот" reflects the same degrees of physical proximity (which is why I'm arguing that 'this boy' is the correct default translation in this case).
My previous message was to illustrate the additional meaning of "этот", which does make it sometimes possible to translate it as 'that', as you indicated earlier. But yes, it would be a bit of a stretch to do so here.
Without additional context I also personally prefer to use "that" in situations such as this when people are mentioned. Assuming they are indeed in close proximity (which we don't know), it seems incredibly rude to refer to a person in such a way. So to me, we're either left with "This sentence sounds rude and unnatural" or "Please add "that" as a translation". I chose the second for consistency reasons.
There is a difference and I also would have marked it wrong though I am finding it hard to explain why. :)
However, you can even see in your own examples that you are using them slightly differently: "to like to do" vs "to enjoy doing".
"Enjoy" has more of a focus on the enjoyment of the process while being in the middle of it. "Like" expresses a more general positive attitude towards something.