"I am her younger sister."
Incorrect. Ane(姉) translates to my older sister. You use this when talking to others about your older sister. Oneesan(お姉さん) is used when talking about other people's older sister. You can also use Oneesan(お姉さん) when talking to your older sister, like calling her older sister instead of my older sister.
Sorry if that ladt part aas a bit confusing.
Im no native but I have read around on this and flexible word order in other languages and its almost always this. Yes the order is flexible and the sentence will carry the same literal meaning but the placement of words depends on what new information you are trying to stress or there is just a natural way that people usually talk and deviating just sounds odd.
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."
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 は dictates the things preceding it are the topics. Particle の dictates that the things preceding it "own" the following things, like putting an 's on a noun in English. "Maria's book" is マリアの本 & "my book" is 私の本. So, what you're saying is kinda like: "As to my she, is little sister." I say this with confidence. I've been using Duolingo, and other apps/software to learn Japanese for years.