Off the top of my head I would say the marquez brothers along with the klits are the most talented brothers.
I think genetics do matter a lot though. I mean it may seem like there is a huge difference in skill between the family members but there are quite a few boxing families with multiple world champions amongst them and only a very small percentage of boxers reach that level.
Also things like reflexes can be worked on some but for the most part people are born with them. And you look at guys like the klits who were blessed with good genetics and look at the mayweather family where mayweather sr is like almost 60, with that lung disease and still has a 6 pack. Freaking roy jones would still have a six pack if he had an office job and did curls twice a week.
People like to believe that its mostly hard work and dedication because its a nice idea that anyone could be a champion if they just trained hard but the reality is that genetics do matter a lot and life isnt fair, everyone isnt equal. Some people are born smarter, more athletic, better looking or born into wealthier families than others. Its just the way it is.