For a starting definition so there's no confusion
Definition of incest
noun
sexual relations between people classed as being too closely related to marry each other.
Origin:
Middle English: from Latin incestus, incestum 'unchastity, incest', from in- 'not' + castus 'chaste'
It's from the Oxford Dictionary.
For discussion:
-Is incest morally wrong?
-In the case of no possible offspring?
-With offspring?
-Should incest be legally banned?
-Does the act of incest disgust you?