How Hume employ the fact of animal incest to advance his argument that morality does not consist merely of “matters of fact”

How does Hume employ the fact of animal incest to advance his argument that morality does not consist merely of “matters of fact” and that morality is not merely an “object of reason”? Explain Hume’s idea that morality is a matter of feelings and sentiments. Why does Hume have a problem with deducing an ought from an is?