"I am her younger sister."
I think you can say 姉（あね）because it's YOUR sibling. However, maybe it's different if you're talking about her in third person.
Yes, this is intentional. Occasionally you'll need to use single kana to form a word needed to complete a sentence.
"Her younger sister is me". Yes, that's right, but it's as awkward-sounding as it is in English.
The は particle would indicate the topic, and roughly translates to, "as for (blank)." So this sentence would be, "As for her younger sister, that is I." I think in most cases that wouldn't make much sense. Usually you would say, "As for me, I am her younger sister." Perhaps if someone asked you, 「彼女の妹はだれですか?」, then perhaps it would make sense to say, 「彼女の妹は私です。」 but even then I think it would make more sense to say something like, "[I] am her sister," rather than, "[Her sister] is I."
Well, I did かのじょのいもうとです。 I think that just means "is/am her younger sister" and with the implied subject thing it works, but I'd like to know there's anything seriously wrong with it. If I said that in Japan regarding my own sister, would that be understood?
I answered likewise and was judged correct by Duolingo. I am not (yet!) an expert, but since the grammatical construction is exactly that of basic declarative sentences (I mean, « です »), my guess is that it is fine.
That means It is my sister. You need は because is denotes that you are the subject and not your sister.
Kanojo no imouto desu. She (possessive particle) younger sister is. My, his, and her are all stated by using I, he, and she and then the possessive particle. Watashi no, kare no, kanojo no. It's actually quite a basic sentence owo
私は is closer to "As for me," because は would indicate topic. The "am" part would be です 。
I'm still a beginner in Japanese so i may be wrong, but i think you wrote "Her me is a younger sister"
I am her younger sister. Watashi wa = as for me Kanojo no imouto = her sister. Since you are talking about you, you should start with watashi wa. Im not good at english and a japanese beginner as well. I dont know when are they going to introduce grammar correctly but this would be easier for you if you know what は and の mean
The subject of the sentence is you (I am)[watashi wa...desu].
They call this the topic when we're in Japanese. The girl is the object (her) because we're talking about its relation to the subject. The relation between these (older sister) is described by の which would be added if you rewrote the English sentence to read "I (わたし) am (は...です) the girl's (かのじょ の) older sister (いもうと）."
Particle の shows possesion over something. E.g. わたしのかさ — my umbrella, かのじょのなまえ — her name.