Here is what Juvenal writes about Domitian's relationship with his niece:
From Juvenal Satire II
If Clodius condemn adulterers, or Catiline upbraid Cethegus; or if Sulla's three disciples inveigh against proscriptions? Such a man was that adulterer who, after lately defiling himself by a union of the tragic style, revived the stern laws that were to be a terror to all men -- ay, even to Mars and Venus -- at the moment when Julia was relieving her fertile womb and giving birth to abortions that displayed the similitude of her uncle.
This was incest, a stuprum, by a piece of Augustan legislation, lex Julia de adulteriis coercendis, which Domitian supported. The Younger Pliny in Book 4, epistle XI writes about this, as well.
From "Domitia Longina, Julia titi, and the Literary Tradition"
Martha P. Vinson
Historia: Zeitschrift für Alte Geschichte, Bd. 38, H. 4 (4th Qtr., 1989), pp. 431-450.
"Domitian is, in a sense, hoist with his own petard, for had he in fact been guilty of incestum as charged, under the lex Julia de adulteriis, to which Juvenal alludes in his account of the incident, he too, like Licinianus would have been subject to relegatio ad insulam and, as a consequence, political ruin. The accusation of incest against Domitian thus serves retroactively to impeach the legitimacy of his regime by imputing to him a crime conviction of which spelled disqualification from public office. The unstated but nonetheless inescapable conclusion to be drawn from the Julia affair is that Domitian's overthrow was not only justified, but long overdue."