Spider Baby or, the Maddest Story Ever Told

The credits dub this "the maddest story ever told," a promise that's well on the way to being fulfilled in the opening scene alone, when Virginia traps and kills and hapless deliveryman in her makeshift web. She's one of three siblings who suffer from a unique genetic disorder that causes them to regress back to childhood, while retaining the physical strength and sexual maturity of adults.