"I am her younger brother."
Wrong. "I am her younger brother." = "Je suis son petit frère." in French. The verb is not at the end in French. This sentence's grammar is pretty very same as English. If the sentence was negative, then it will be: "I am not her younger brother." = "Je ne suis pas son petit frère." Only different is that extra word "ne" before the verb.
It's proving a difficult unit for me too. A lot of things aren't clear and they are asking us to construct different types of sentences with different sets of particles in different ways. I'm sure this is simple to more experienced learners but for people still trying to understand when to use は, の and が, as well as remember which kanji we're suppose to use at this point, it's pretty daunting.
は is for 'is'. John is student. The の is used as posessive. My chair, his sister. And が is used when the subject is being used and isnt the actor. Like: The chair is bought (chairs dont buy things). Anyway, Japanese isnt a hard language. Barrier to entry is the writing though. Before this i focused some time to Remembering the kana / Remembering the kanji. So focus on the early lessons first. Reading comfortably makes focussing on the rest easier.
Please correct me if I am wrong:
ka re ra - They (male)
ka no jo ta chi - They (female)
ka re - He/Him
ka no jo - She/Her
o ni i (san) - (someone else's) older brother
o ne e (san) - (someone else's) older sister
o ni - (my) older brother
o ne - (my) older sister
o to u to - (my) younger brother
i mo u to - (my) younger sister
o to u to (san) - (someone else's) younger brother
i mo u to (san) - (someone else's) younger sister
shi ma i - sisters
kyo u da i - brothers
Couldn't you just say 「かのじょのととうです」 ? Why is there a need to affirm that you are the subject of this sentence? I understand that it would be highly sensitive to the context of the conversation and it might as easily be understood as "That's her little brother". Maybe I'm being a bit too difficult.
Yes, you could say that. It's indeed likely to be interpreted as "It's her younger brother", but since it is so context-dependent it could technically also mean "I am her younger brother", without including わたし. Be careful with the spelling of little brother though: it's おとうと (弟)
If you want to sound natural dont use watashi.... btw ore can come off as rude while most male adults dont use boku...
EDIT: I have read more about this and this is what I have found. According to an article I read by a non-native speaker who has lived in Japan, he claims that BOKU is used by males and some females while WATASHI is more formal as you said. But this "formal" means that with STRANGERS you would use WATASHI (Female or male).
ORE is used by males but only among friends.
Now based on what I have observed in J-dramas and listening to J-Pop, I would say that the observation about ORE is correct but I am not so sure about BOKU. I still have to hear a male using it in any drama I have watched and I have only heard women or kids use it in dramas and songs. Males always use ORE or WATASHI.... There are other forms of "I" and this can also vary by region as well which is why "WATASHI" is the best one to use for beginners....
Hope that helps! (Notice that my original suggestion to drop the topic marker is the best if you want to sound like a native)
It's similar and grammatically possible, but it sounds strange and (just like in English) you'd be changing the topic/subject of the sentence.
To break it down: わたしは = "As for (about/regarding) me", かのじょのおとうと = "her younger brother", です = "I am" (in this context). On the other hand: かのじょのおとうとは = "As for (about/regarding) her younger brother", わたしです = "it is me".