The blood relationship doesn't matter as much as the familial one.
It is a betrayal of the safety that a person should feel within the family. And it is profoundly damaging to the kid and to any siblings as well (because they are afraid they may be next).
Usually if you take a good look at traditional morality it has some very sound reasons behind it.
You are too kind, Mother. If you look at ANY kind of morality or cultural compact is what you meant to say. This is the age of nihilism. Everyone is their own god. The natural law is up for debate.