The petition to annul the marriage to Catherine of Aragon was based on a law that forbade someone marrying their brother's widow. She did not commit incest (that would require a brother or father being involved) but the dilemma for the Pope was that the marriage had been granted dispenation to go ahead in the first place based on the claim Catherine and Arthur had not consumated it. Suddenly Henry wanted it annulled for the converse reason. While the dispute dragged on, Henry knocked up Anne Boleyn (he was shagging her AND her sister, what a trooper) and he decided that the Pope was a wanker and declared himself head of the Church in England, annulled his marriage to Catherine and married Anne. Ta-da. The incestous affair was between Katherine Howard and her cousin Thomas Culpepper -- although I believe doing your cousin is perfectly acceptable in some parts. Make a great soap opera, hey?