Security clearance will include family members, and it's entirely possible someone will be denied clearance because of something they've done that makes them susceptible to coercion. The candidate would need to disclose things like this.
Reddit is full of examples where someone's family impacted their clearance (either denying it or requiring disclosure):
> It's entirely possible someone will be denied clearance because of something they've done that makes them susceptible to coercion.
And that doesn't have to be something incriminating, either, just something that means they could be used as a lever against the individual seeking clearance. An example might be something like having a family member who lives in Russia - this would make the subject susceptible to coercion by the Russian government threatening to harm their family.
Isn't that the case for any government employee?