What does the narrator's mother believe to be his responsibility to Sonny?

The narrator's mother believes it's the narrator's responsibility to look out for Sonny.... to make sure he doesn't get into trouble..... to make sure nothing happens to him.

"You got to hold on to your brother," she said, "and don't let him fall, no matter what it looks like is happening to him and no matter how evil you gets with him. You going to be evil with him many a time. But don't you forgetwhat I told you, you hear?"