You're a cult member.

From an old god cult.

You were supposed to babysit your younger sibling.

Something happened and now your sibling is possessed or had their mind eaten or something similarly unfortunate by the old god you are all worshipping.

You could probably tell your parents and they'd see it as an honor.

But. Then you would have to admit you botched your first babysitter job.

Cue attempts to hide the fact that your baby sibling (age optional but preferably between toddle age and puberty) is no longer your baby sibling but an eldritch entity possessing their body.

Can go the horror way and be kind of sad and philosophical or take a more lighthearted and comedic turn with ever sillier and more ridiculous attempts to hide the facts.

The protagonist can be of various age, like in their teens, which would serve as a good explanation for their refusal to come clean, or older than that and just very determined to keep this whole mess a secret.

As in, really determined.

The world can perish but that secret they'll take to the 

grave determined.