It doesn't mention anything about maternal incest whatever.
And the historical fact is that almost everything we hear about John XII's personal character comes from the contemporary account of one Liudprand of Cremona.
Who was John XII's mortal enemy? Emperor Otto of Germany.
Coincidentally, of whom was Liudprand a personal employee? That would be . . . Emperor Otto of Germany.
What little we do know from several sources about John XII's character is that he cared more about hunting and political wheeler-dealing than about the responsibilities of his office, and that the local Romans had zero regard for this French-sympathizing outsider and loved to gossip about him.
What there is zero evidence for is maternal incest. Not a shred.
This very much, in a common sense way, flies in the face the claim that Rome's primates are the sole "vicars of Christ" or that "ex Cathedra" (however you might define that...) they speak for God.
It also, again in a common sense way, throws a monkey wrench in the idea of "uninterrupted" apostolic succession if the "see of St. Peter"....even if only 5% of Boetner's alleged histories are true.