"I am his younger brother."
The natives would always use the kanji, if you write everything in hiragana it gets difficult to read because of the lack of spaces. The Japanese keyboard suggests you the kanji when you write the word in hiragana or romaji (in the gboard keyboard you write in hiragana like on the old cellphones, in the swipekey keyboard you write in romaji and it turns each syllable automatically to hiragana)
Yeah, the second structure is like saying "His I am younger brother". It's difficult to explain, but it's how a sentence is structured in Japanese. Usually the subject (if it's mentioned; in this case, "watashi"), will be in the start of the sentence. Another thing is that "kare no" and "otouto" should be together because "kare no" is working as the possessive case that indicates that it's not someone else's brother, but his brother.
Correct me if I'm wrong, but to me this seems like it would be translated to "Speaking of his younger brother: that's me, by the way." If 僕 and 私 can be used like that (which I don't know), it would be useful in a conversation where e.g. the younger brother just enters while his older brother is talking about him, but I can't think of any usual situations where you would say it like that. :)
So you're implying it was marked as wrong? Because it actually should be, you're right. In any situation that I can think of, it should be clear from context what you're saying. Though, I could imagine that Duolingo got a problem with you writing that in hiragana, as I've read in these comments that it's rather unusual to include the お, unless you take the kanji, as the kanji includes the o by itself. I did not exactly get that myself, but I would recommend just using the kanji whenever possible, as that is one of Duo's strong points: It always prefers kanji, as Native Japanese people would do.
If I'm not wrong, or leaving something out... I believe it's something like this:
Masculine: 僕（ぼく） Is used men and young boys. Common. 俺 （おれ）Is used by men and can be rude at times.
Feminine: 私（あたし）Is a common way of saying "I". 内 （うち） Is sometimes used by young girls
Gender-less/Used for either: 私 （わたくし）More polite/humble (Not really used for everyday - more for those in the service industry or business situations.) 私 （わたし）Common so safe to use.