this ....

from Ken
99 USC-White, of course. Why do you ask?
although, I prefer 99a USC-McCarthy (superceding amendment).

OK, chasing the "Fruit of the Poisonous Tree" is illusory here cuz it does not apply unless the statements are offered into evidence (e.g., UCMJ provision cited by Ken).

What could apply are the declaratory US legislation regarding the scope of Common Article 3, the separate War Crimes and Anti-Torture statutes, and other provisions of the UCMJ - as to all that, we've been there and done that in "War Crimes" and other threads (the Search button is your friend).

As Ken correctly states:

.... intimidation is a misdemeanor most places and in some a felony if weapon is used or harm results. Last time I heard, anyway. That could well apply even if her custody was not solely due to the relationship; intimidation is intimidation.
Even the original hypothetical (construed as a pure ruse - i.e., no intent or ability to carry out the threat) runs afoul of the prohibition against employing a death threat (against the detainee and/or his family).

So, legally (that is, if I am advising the interrogator as my personal client - which is not quite the position of a JAG officer) the original scenario is questionable. My enhanced scenarios are more so. Witness the court martial of the CPT and his 1SG (thread - A War's Impossible Mission).

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True, if you assume (accept without evidence) ....

from LawVol
Operating under an assumption, given the limited facts, that the wife and sons are civilians they are accorded protected status under international law. Once this status has been determined, they cannot be held and, thus, their continued detention becomes illegal.
but, the issues under Common Article 3 are not so simple as making a distinction between those who bear arms and "civilians". The question of who is a member of the infrastructure (the underground cadre) and subject to CA 3 detention is not settled by automatic reference to GC IV for everyone not caught with an AK or grenade in hand.

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This scenario reminds me of something biblical (or Koranic) I once read - canonical or non-canonical, I can't remember. Anyway, the punchline belongs to Schmedlap.