First of all, St. Catherine of Siena, though she was a great saint in many ways, is no "Doctor of the Church", for she was never part of the Ecclesia Docens. But that's a different issue.
Secondly, St. Catherine's revelations are in no way part of the Church's magisterium. If Our Lord intended to reveal this as Church doctrine, He would have done so in the Sacred Scriptures or through Tradition, not through St. Catherine "dialogues"--which, her personal sanctity notwithstanding, could have been the fruits of her own active imagination.
We have quite a few mystics, who though renowned for their personal heroic virtue, actually contradict one another at times in the content of their "revelations." Perhaps there's a reason why it's almost always women who have such revelations.