The comments from Balance1771 state for me times it could be even remotely considered allowable- consenting adults, mentally fit, no power plays/coercion involved. Even with consent and no psychiatric malady, I am not particularly for it and there are far too many available men and women on the loose in the world to make for the necessity of this.
Once more mitchymo, you present incest and give no age parameters or other details. In fact, in this post, the one immediately above, you say 'any incest'. Really?

You mean such as a 4 year old and an adult?