Twisted Logics in "Tis Pity"

The play opens with Giovanni trying to convince the Friar that his desire for his sister is, logically speaking, very natural, even appropriate, that their blood closeness would only make their carnal closeness more perfect (or vice versa) and that it is mere social / religious convention ("customary form" 1.1.25) that keeps brother and sister apart. The Friar doesn't really have a good response to Giovanni's argument (presumably because The Bible seems to both condemn and condone incest).

Later we see Soranzo twist his way out of his promise to marry Hippolita by arguing that to keep a sinful promise is more sinful than to break it (i.e. two wrongs make a right).

Still later we see more twisted logic when Grimaldi slays Bergetto and the Cardinal sanctions it because it was a case of mistaken identity - Grimaldi meant to kill Soranzo so no harm, no foul.

Given the way logic is used (or abused) in this play, particularly in reference to the Church and morality, what is Ford getting at? Is Giovanni's argument about sibling love (despite being a bit icky in premise) valid in the face of the Cardinal's sanctioning of murder and Soranzo's selfish abandonment of Hippolita?