This is my first discussion, and I want to share with you my point of view.
The question is, when we are allowed to put in chastity someone else.
I will take the question under two important aspect: the moral one (of course subjective, but I will give some rationals here) and the second, legal (I will provide framework information based on western view, because of course I don't know every single legal system)

Being all equals, the title needed to put a chastity belt on someone else is consent.
Consent, to be correcty given, needs five main characteristics

informed
freely given
Clearly stated
given by someone who is accountable
Can be revoked anytime

Why? Because since putting a chastity belt affects body rights and sexual freedom, two basic human rights, it must be freely given in a relationship between equals. NOTHING ELSE IS ACCEPTABLE. All information must be provided, consent must be free, it must be clearly expressed, must be given by someone who is capable of discernement (not minor, not diminished capacity), can be revocked without consequences.

I will take Milady, my wife, as an example. She has all the ability to pledge herself in chastity. But what if she has economic problems and he's unable to stand by herself? Can I push her in chastity? Obviously not, because there will be no free consent.
And if there is no consent, there is abuse.

Now, it's pretty simple to understand that putting a chastity device on a offspring is ALWAYS WRONG
On a minor, because it misses on point 4, but also on adult sibling, because its not a relationship based on equally adult, but it lacks the freedom characteristic. and remember: Where there is no consent, there is abuse.
That's why enforcing sexual activities (and putting a chastity belt is a sexual activity) on someone who is clearly disadvantaged is ALWAYS abusive. Same apply to legal guardian.
And what about the law?
I'm pretty sure there is no law preventing or allowing explicitly the use of chastity devices, but most jurisdictions consider them sex toys, so using on a minor can easily be considered sexual abuse and on adults if it is proven that it's not willing (not a difficult thing: not having the key, they can't get rid by themself) its sexual assault.
I'm pretty confident that most people that write here, should they decide to ask legal help, can very easily sue their keyholder and probably also have them incarcerated for sexual abuse.