yes there's many sentences in many hadiths that use that pharase 'bro' or 'sis'. But it's a term used normally when you don't know the name of a person, for xample you have a hadith where the prophet called someone "Ya Akhal '3arab" (Oh Arab Brother), infact, more commonly the prophet would call someone who he didnt nkow their name "Ya '3abdallah" (Oh servant of Allah), and he would call to a female 'Ya Amatallah' (O servant of Allah - amah is arabic word for servant).
And the companions use to call themselves by their nicknames (like Abu Hurayrah "Father of the kitten", Zayd-Ul-Khayr "the Zayd of good") or their full names (like Ali, Hamza, Umar), and the prophet use to adress people by their names. In fact i remember reading in hathal-habeeb ("This is the beloved" - very neat book) when Ali was describing the behaviour of the prophet to people who havnt met him, he mentioned how he use to face people when he spoke to them, and he use to call people by their nicknames. All this makes the person who is being adressed feel that htey are being valued by the prophet.
Infact, you are right we use the word bro/sis more than we do label people by their names or nicknames as was teh Sunnah of the prophet. But we definately are brothers and sisters and there's no problem in labelling each others so, its better than saying 'hey fella come here'.
