As far as I'm concerned Denji dating Power would be basically incest and I'd imagine he'd see it the same way. That's his little sister, his family, not an object of lust you heathens.
@Jetsetlemming Yeah, that'd be absolutely unethical to put that pseudo-incest into a pure and chaste story that features body-horrors, oceans of blood and gore and some romantic cannibalism... Yup, totally not Fujimoto's style.