Translation:I am his younger brother.
Yes. You are right. But 'o' of 'otouto' is part of the noun. not honorific meaning.
And younger siblings are younger than you in your family. So they are commonly called their names or nicknames by family.
People who are not families say '弟さん' and '妹さん' about your younger brother and younger sister. In this case too, 'o' is not used.
Actually, in my experience, I would say that 俺/オレ (おれ) is much more common for men in casual speech. Older men will even get away with saying it in formal situations.
From what I gather, 僕 (ぼく) sounds somewhat boyish/childish, though it is more likely to be used in formal speech than 俺.
かれ and かのじょ are not more commonly used to mean boyfriend/girlfriend in my experience, though it is not uncommon to hear them used that way. In fact, I think かれし is a more common way to say "boyfriend", moreso than かれ.
I also don't think these sentences are unnatural or convoluted at all. Rather, trying to find other ways to structure them to not use かれ or かのじょ feels much more convoluted because all the ways I can think of would be heavily reliant on constructing the right context.
あなたたち is a plural "you". For 'they' either 彼ら(for all boys or mixed group) or 彼女ら/彼女たち (all girls).
The と particle is only really used when listing nouns. Listing verbs/adjectives/phrases you would conjugate them into their -te form, and leave the last verb at the end in their normal conjugation. In this case the -te form of です : で. (I believe the -te form is introduced in the Activity 2 skill)
You also don't need to repeat "I" as the topic. It is usually known from context that the topic is the speaker and can be dropped entirely.
(optional "I") (Their older sister) (am and-) (His younger sister) (Am)