This is the good stuff. 9.5/10.
Two gender-noncomforming siblings meet an ancient cursed snake-lady in a forbidden forest, decide she's their best friend now, and make a point of involving her in their lives from then on. Wholesome horror comedy with good world-building.
IMO it just needs a little extra something to take the step into the full 10/10, like an iconic idea or a truly good quote, or a joke that will take up real-estate in my mind for years, or an elegant callback, like the siblings using their laser-beam SFX to solve a real problem, or a splendid plot twist. The author clearly has the chops for it.
But yeah, this manga definitely will make it to what I define as "The All-time Classics".