Now I realize they were trying to decipher the sounds of individual words while I was focusing on my brother's intonation patterns, body language, and facial expressions.
Babies don't all learn to talk the same way. Some start with individual words like "Mama" and gradually add one word at a time to their vocabularies. Others, like my brother, focus on the intonation patterns, etc., although they usually include a few intelligible words in their sentences and the number of those increases as they learn.
I wonder if babies' temperaments influence how they begin to communicate. It's not very scientific but, in my limited experience it seems like the little ones who began trying to talk with intonation patterns before using clear words became especially sociable, outgoing, and friendly children and adults. Maybe those personality traits made them more aware of the emotional side of communication.
Even if someone were able to collaborate that tendency by a scientific study, it wouldn't be able to show which was cause and which was effect, so I guess I'll never know if my theory is correct or not.