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I think he knows what her reaction might be. Holmes mentions to Watson that he thinks it was strange for Mrs Douglas to have shown so little outward emotion and not to have rushed to her husband's body.Interrogation of the house servants reveals little, and after Mrs. Douglas, wife of the murdered man, and Cecil Barker, a friend of the family, are questioned, Holmes reveals to Watson that he believes the two are in a conspiracy together.
Barker did not want Mrs. Douglas to see the dead body because her husband's body condition was not good. She may get upset or may succumb.