If you put it that Mrs.Douglas and Barker know the truth about the murder, and are conspiring to conceal it, then I can give you a whole-souled answer.
I am sure they do.

But your more deadly proposition is not so clear.

Let us for a moment consider the difficulties which stand in the way.
"We will suppose that this couple are united by the bonds of a guilty love, and that they have determined to get rid of the man who stands between them.

It is a large supposition; for discreet inquiry among servants and others has failed to corroborate it in any way.

On the contrary, there is a good deal of evidence that the Douglases were very attached to each other." "That, I am sure, cannot be true," said I, thinking of the beautiful smiling face in the garden.
"Well at least they gave that impression.