When it comes to seduction and incest, you have a zero percent chance of success if your target is not already amenable to incest. If you do not belong to a religion that is OK with consanguineous marriage, the target must not be very religious (i.e. has to have a lower than a certain amount of zeal) and also has to have a trait that lets them be open to unorthodox and/or taboo sex things. This is also all on top of general heightened requirements for seduction, where the target needs to also be romantically interested in you for you to succeed (generally measured by a combination of personal opinion, certain opinion modifiers, and/or certain traits).