What's the title of the Poirot mystery where the "murderer" does not himself murder anyone?

Question

I vaguely recall a Hercule Poirot mystery where the "murderer" does not commit a single murder but he manipulates other people into committing murders. I believe Hastings' niece or some such relative also appears in it.

Anyone remember the title?

Thanks in advance!

Answer 1

Oh, gosh, I think it was the very last Poirot book, because while Poirot knows the man is responsible the law can't do anything about it. Curtain was the title. Does that sound right?

I found a synopsis: http://en.wikipedia.org/wiki/Curtain_(novel)
Hasting's daughter has a role in it.