HP & the Half-Blood Prince Came Out Yesterday!!!!!!

I finished the book last night, and I must say that I am amazed. This is, in my opinion, the best book that our favorite authour has written yet. I will try my best not to give anything away, for those of you that have not read or finished the book yet. I must say, the ending shocked me. Who would have known? For those of you who read the book, I think that the mysterious R.A.B. just might be Sirius's little brother. I'm checking the 5th book. Any other ideas on who R.A.B. is? Post it here please.