In what ways did Marguerite consider she had sinned?

In Chapter Nineteen, Marguerite says that the worst sin she's committed is in failing to recognize her own husband.

She had done—unwittingly—an awful and terrible thing—the very worst crime, in her eyes, that woman ever committed—she saw it in all its horror. Her very blindness in not having guessed her husband's secret seemed now to her another deadly sin. She ought to have known! she ought to have known!