To tell you the truth, I trust my own experiences, from closed courtrooms, special facilities set up to interview child rape victims, and neglect/abuse files, to tell me that incest is indeed a real and devastating phenomenon. I trust statistics less, generally keeping in mind the saying, "There are three kinds of lies--lies, damned lies, and statistics."
For example, what about this? Studies conclude that 43 percent (43%) of the children who are abused are abused by family members, 33 percent (33%) are abused by someone they know, and the remaining 24 percent (24%) are sexually abused by strangers. Isn't there some overlap between the group who are abused by family members, and the group who are abused by "someone they know?" I mean, a family member would also be "someone they know," wouldn't it?
Also, I don't see how a child who is sexually abused by "someone s/he knows" (without more, to indicate a relative), or a stranger, is a victim of incest. Strangers? I mean, I guess someone could meet a family member and be molested by him on the same day, while he is still sort of a stranger, but isn't that stretching it a bit?
Also, I wish the source had broken down the group "family members" further. A family member can include step-parents, step-siblings, adoptive relatives, uncles and aunts, in-laws, and on outward to more distant relatives. Would you two at least agree that incest perpetrated by a full-blood close relative such as father or brother is at least less common than incest perpetrated by a more distant relative, or a step-relative? The girl in question lived, as far as we know, only with her closest blood relatives. Presumably incest, when it occurs, would occur more frequently among relatives who live together, since there are far more opportunities. Yes, of course it happens. And I'm not saying that a close relative such as a father or brother might not do it. But what do we know to indicate that it happened to Elizabeth?
Since it is so hidden and under-reported, why would you even think the public would know? Even mothers and other family members often keep it hidden to preserve the family reputation and with hopes it won't recur, with counseling, etc.