Yeah, you may be referring to those supposed 'tribal/native people' that inhabit the outskirts of the village/town. We may learn more about them in the next update. They for sure have some kind of history with the townsfolk.
Moreover, I'm also guessing (i.e., assuming that the MC's sister is still alive and not merely a figment of his imagination), that those native people probably have captured and/or lured the MC's sister to take part in a ritualistic rite/ceremony of some sort. Or perhaps they view her as a deity. Who knows? Either way, the storyline of this VN is quite interesting. Gives you a reason to get invested in it, and not merely just for immediate fap material.