You always infer it from context, even in English. In the sentence He remembers his brother, it is even possible that "his" does not even mean belonging to the subject "he". Theoretically it could be another male's brother, but that would only occur to us if that was what the context suggested. In Italian, the third person possessive pronoun would be assumed to refer to the third person subject unless context dictated otherwise. But obviously with more possibilities of meanings, context will dictate otherwise more often.
That's interesting. The only thing I know about Finnish is that it's not an Indo-European language and that it's related to Estonian. But since I don't know Estonian, the latter doesn't help. I speak a fair number of languages, but they mostly are Germanic or romance languages. It's a rather small corner of a very big pool.
It is not uncommon to hear the wrong thing, even in your native language. People do misspeak or enunciate poorly, and sometimes you just hear wrong. But it's important to learn the clues that a native speaker uses to "correct" what he hears. This sometimes happens so quickly and naturally they aren't even aware their brain made a correction. This is an easy case. Fratello is always a masculine word, and there is no word fratella. So that not only means that sua fratello is always wrong, it means we know the error has to be with sua and not fratello. This is clearly a place where an Italian speaker would self correct what he heard without thinking twice.