Well T-S, one of the benefits of a Captain using his (or her) discretion, is that they can actually profile the person about to enter their domain.
Unless the FO is an immigrant from a particularly militant region of North Pakistan or insert your own high risk area here it seems highly unlikely that the parents are suddenly about to take the jet down (perhaps same odds as getting hit by a meteorite while in mid cruise), and even in the case that said FO originated from a troubled area, more likely that if he/she had been turned, would do the job alone rather than bring their parents in to it...On the other hand if they were inviting their younger (potentially hot headed) brother.....well you get the point
And before the pedants start flaming the example, its designed to be a provocative one, as there are potentially millions of different combinations, many with nothing to do with terrorism, where risk is profiled and measured in individual cases on a day to day basis, by educated people.
You cannot eliminate risk, only reduce it, if you try to eliminate it completely the only result is you end up in wonderland, living next door to Alice (Alice, who the **** is Alice ? :-)