What does the narrator's mother mean when she says that William Armstrong might not be "all there"

The narrator means that he might not be all there mentally. Born with a heart condition, it was believed that he wouldn't survive infancy, let alone grow. It was expected that he would be both mentally and physically disabled. Fortunately, although William would live a very short life..... he was intelligent, talkative, and in his brother's words, "talked so much that we all quit listening to what he said."