He is scared. Certainly, his brother is annoying and irritating and general nuisance all the time but he can't just leave him in the middle of this scary forest, right? And anyway, his brother is just eight years old is frightened of everything that moves in the dark. He has to find Reggie.
Their parents had taken them on a country trip in Hampshire and Sirius decided to camp in the forest and Reggie tagged along, because he mimics everything Sirius does. But then the sun set and it became dark and they realised that maybe camping wasn't such a great idea after all. So they ran home but of course Reggie had to do something stupid so he is now lost. And Sirius must find him, hopefully alive.
The weather is becoming colder, and Sirius is shivering. He wishes he had snuck Mother's wand along with him and cast a warming spell. Not that Mother would have left him in one piece after that, though. But it would have been worth it.
'Reggie! Where are you?' he shouts, his panic level rising. 'Regulus Arcturus Black come out right now! This is not funny!'
'Sirius?' he hears Reggie's voice from somewhere near the clearing. He runs towards its source. And right there, surrounded by the remains of their campfire and half-dismantled tent, sits his stupid brother.
'Why did you come back, you dimwit?' he asks, hands on his waists. His brother looks up and gives him one of his stupid smiles. 'I knew you'd come for me.'
Sirius promised to never leave his brother behind, ever.