You've probably already read about the MacKenzie Phillips memoir in which she reports experiencing incest with her father. For ten years.
This component that "it started out as rape but became consensual" is something that I think is confusing people. This is something that I have seen in rape victims as well as heard second-hand in similar published accounts. One way to make sense of this is to remember that rape is one of those crimes of power, oppression, and possession.
So is kidnapping, for example. You probably already know about Stockholm Syndrome, which is basically a survival mechanism that enables victims to--well, survive--in an untenable situation. Similar processes can be seen in rape victims. It is not uncommon to see a woman dating someone who initially raped her. She may or may not have recognized or labeled it as such, even to herself. However, it is still damaging and traumatic--and continues to be damaging and traumatic.
In a culture that rape-ifies sex and romance, and glamorizes rape, I see this kind of acceptance of rape as a valid relationship basis as a variety of Stockholm Syndrome. If we did not live in a rape culture, then women would be much more likely to immediately recognize that what was happening to them was unacceptable, but as it is--it can be hard to set limits when a person believes they aren't allowed to, or that things are supposed to happen this way. Ms. Phillips' experience does not sound far-fetched to me at all.
The thing about "having a needle in her arm for 35 years" [paraphrased] does not reduce her credence for me, but rather increases it. The more various dysfunctions exist in the family system, the more believable others seem. Who the heck wouldn't be using drugs and alcohol in order to deal with the weirdness of her situation? These things don't exist in isolation.