Has anyone read "The Doctor's case"?

It's a short story about Sherlock Holmes in the Nightmares and Dreamscapes book.

I don't understand the ending. They figures out that the whole family had something to do with the murder. But than Holmes says they all would have died in six months anyway. And than they tell the police it was a robbery gone wrong and no one is arrested.

I don't get it. Would someone explain it to me?