Just be careful. Sure, he's chill RIGHT NOW. If you start dating and u piss her off he could become a problem.
Remember, girls tell best friends just about EVERYTHING. ESPECIALLY when things start going wrong with the men they are dating. I don't know if u have a sister but just IMAGINE the things she could end up telling him if the dating turns bad and imagine how a big brother(because he still IS her brother, best friend or not) would react. Maybe nothing will go wrong. Maybe there is NOTHING to worry about. Just make sure you think of all of the possibilities and decide if it's worth it. Good luck.