Real Scarecrow Hours

In the comics, Farkus is stated to already have kids when Nihil is about 18 years old.

For my own worldbuilding, the following order of events would make sense: Sister creates the foundation of the Ghurch as we know it / early Ghost Project ⇨ big falling out between her and Nihil; they likely lose touch for some time ⇨ the cult continues functioning, duuuuhhh, but the band is forced into an indefinite hiatus ⇨ the Project is brought back as an excellent propaganda tool—the one issue is that the Ghurch still needs a face to pull it together publicly ⇨ solution? Primo's older Satanic fanatic of a brother; doesn't matter how many children he might have had, or what he's been doing up to that point—we all know that Primo would throw everything aside if it meant serving the devol with all his life...

I like to think that Secondo and Terzo have spent the first years of their lives outside of the Ghurch (or mostly outside of it? perhaps while being loosely monitored / having kept tabs on them?). In contrast, it'd make sense that Primo=Farkus (in this case, their uncle) turned out so fanatical and spiritually devoted (in an old-school, "mirror of the stereotypical devoted clergyman trope" way) due to being a cradle-cultist.

Sure, we haven't seen Marika or Nihil behaving fanatically, but that doesn't really matter; the same upbringing often leads to different outcomes. I wouldn’t be surprised if Primo spent the time away from his family on doing his own religious research and trying to deepen his connection to the Infernal.