well guys um back with another guess of who R.A.B...is...
after reading the 5th buk thouroughly i realised that R.A.B is Regulus....A for i dun no maybe sirius fathers name lyk harry's is Harry James Potter...so
R.A.B. must be Regulus sumthing for A (his fatherd name).. Black..n do u realise although sirius mentioned his father he never mentioned his name??n i also thought that since regulus died quite a while ago..sirius said that regulus wanted to give up working for lord voldemort...n he died although not being murdered personally by lord voldemort...but i personally think he died after he destroyed the horcruxes n was then killed...maybe all we can do is keep guessing...ill c wether i can find out more about it...