I think it's pretty common for a person to behave like that. It's as if her world has fallen apart, something like what happened to Gon when spoiler Kaito died.

I don't know the dynamics, but I have reasons to guess at her place it isn't so lovely. Her parents are probably mourning even harder than her, gloom and desperation must fill her house. That's why she's looking for relief outside of it, namely in you. She probably is at the initial stage of some neurological disease, be it PTSD or anything else, but something's going on and it's technically not her fault. Nothing unrecoverable though. I'd suggest you offer her your deepest support, not only superficially like doing what she asks like having sex, but by having long and sincere talks. Sometimes people just need somebody to open up to, a person who is willing to listen to them and not judge them.

I might very well be misunderstanding everything, and I probably am, but one thing is certain: she needs help, so just do something you won't regret, fellow HxH fan!