Consider the rat: how you killed it by hucking an old bit of concrete. You hadn't guessed the power of your arm. You hadn't calculated the fulcrum of it, the length it gained this year as you turned thirteen and stretched towards man-size. Everything has felt awkward lately, but not that throw. And now, contemplating a way to find what's missing (a brother--no small thing), you confront the fact that maybe what is required is not a stakeout after all. Rats watch from shadows, but you? You are a killer of rats--an inadvertent killer but a killer nonetheless--which is not such a shadow-thing as it sounds. Concrete: that which can be touched.
Knock and ask. There is no rock her mother can sling so very hard. Go on and ask her where your brother is and why he left.