Power, in addition to being distinctly more feral due to her non-human nature, has the mental faculties typical of an 11–12 y.o. girl at best. If you've had sisters or just remember your childhood friends, you should be able to tell.
Your mind interprets nudity and skinship with a tinge of sexual subtext because you're in the mindset of an adult being in a similar situation with another adult. In such a situation between two adults it would be perfectly normal to expect sexual tension. The only difference here is that Power looks like an adult but she's really not. She's portrayed as being just at the mental age where she'd be somewhat aware of her body features (to the extent of "boys are interested in boobs lol") but is still all-around childish, living in the moment, not very self-conscious about her appearances or behavior, and so on. Reread chapters 4–12 with this perspective, and it should become obvious that this was her character all along.
Denji, who is more developed (but still only about 14–15 mentally), is similarly confused at first when he's confronted with her naked body but quickly adapts to the fact that her behavior just isn't—and has never been—sexually charged, and suddenly being clingy is just her childish way of coping with the trauma. So it properly registers for him as being a platonic bond between close family members, and stops bothering him.
I'm actually quite impressed with the psychological nuance put into this work. I'm sure it will be lost on the core audience of WSJ as it actually requires some life experience to see that teenagers just don't normally have.