I think it is actually a larger subject that has been inadequately explored.

Is it torture to threaten a subject with the torture / rape/ murder of his / her relatives if he / she does not cooperate?

Is it effective torture to perform the above?

This has been performed on Soviet citizens by their government, On Persian citizens by their government, and has been used in many other circumstances. And of course, it is a staple of organized crime. I'm not aware of its use by the US, Canada, or UK.