Demon Father

Don't let the ordinary domestic setup fool you. this goes to genuinely uncomfortable places almost immediately. This 2-episode OVA from PoRO follows two sisters, Airi and Marina, whose mother remarries a man named Kouzou. Marina tolerates him. Airi doesn't trust him at all, and it turns out her instincts are right. Kouzou is a biochemistry professor with predatory intentions toward his stepdaughters, and with their mother constantly absent, there's nothing standing between his desires and the girls. The tension builds around Airi's defiance versus Kouzou's calculated, methodical approach to getting what he wants, including using his academic background in ways that make the power imbalance even more unsettling. The character designs are detailed and the animation holds up well for a 2009 production, and the atmosphere stays appropriately tense throughout. It's a dark, taboo-heavy story rooted in family dynamics gone very wrong, and it doesn't soften that. If you've already watched Oni Chichi: Re-Birth or Oni Chichi: Re-Born, this is the original that started the whole run. If you're coming in fresh and Oni Chichi: Rebuild is on your list, starting here gives you the full context. Based on a visual novel, so the character writing has a bit more depth than you might expect from a short OVA. Go in knowing exactly what this is; it's not subtle about its themes.