Try looking up covert incest.
I'm pretty sure that helping someone with blackheads doesn't need to involve a bed at all, or laying on top of them, AT ALL. I believe your instincts are correct on this, and when you imagine putting yourself in that scenario and feel uncomfortable, it's for a damn good reason.
A lot of the stuff my Dad did was similar to this. It was wrong, but in a way that I couldn't articulate or call out, because it flies under the radar of being openly, recognizably, unquestionably sexual behavior. Abusers are often subtle like that. Like, my Dad liked to play with his genitals, and then play with my hair when I got out of the shower. Like marking me with his scent. When I was 12 I said it was gross and I didn't want him to touch me anymore, and in response I was berated for being a "pervert."
The long-term emotional results of such behavior is apparently similar to overt incest. Did my Dad actually have penis-in-vagina intercourse with me? No. Can I still vividly recall the smell of his sexual organs on my hair when I went to school? Yes. Did it make me feel dirty and not want to sit with my friends? Yes. Is the sense of defilement and violation real? Yes. Flashbacks, shame, confusion, all real. Whether your sister would acknowledge the incident you described as covert incest or not, I bet she has those kinds of vivid memories too, of her Dad on top of her with his face inches from hers and "fixing" her.
Being a "Daddy's girl" is more of a red flag, not less -- I was a Daddy's girl too, it was like Stockholm Syndrome. He doted on me and spoiled me, he was the source of most of my parental attention, and to realize he was abusing me was a truly heartbreaking time in my life. A lot of folks who have been through this kind of abuse have a very difficult time recognizing, acknowledging, and getting help with the problem. A lot of folks who have been through this feel complicit, and feel like they're the ones who are dirty or asked to be sexualized or wanted the attention. Of course we want our parents' attention -- they're our parents. There's a lot of shame and guilt, but it's always the fault of the perpetrating adult who takes advantage of their power over a vulnerable child and that child's implicit trust in them.