Because artist in this case simply don't know how to start things, instead of simply using his imagination, and think about son/sons out there that could realistically have attractive moms, who have this or that weakness, son (or father if other character is daughter) is more horny and excited day by day, female character is having more and more doubts, or more problems or disappointments, whatever linked to real, everyday life, and then slowly (i would always create situation where is male character who is pushing and seducing) they got to sexual part. It could actually be pretty simple, artist simply has to think about possible real life situations. Third party who push them to commit incest is also believable, but it should be more subtle, i guess. It could be therapist-psychologist, it could be moms friend who push her toward her son, or son's friend who is manipulative, or even better, mom's father who is kind of pervert and wants to fuck his daughter, and pushes his grandson to fuck his mom. Many possibilities.