the "why is incest wrong" segments feels like you're trying to make a genuine argument, they go on for a bit too long
for anyone who doesn't know it's because when the DNA of the parents is too similar it significantly increases the likelihood of birth defects, a child with your first cousin is twice as likely to develop a birth defect than with someone completely unrelated
as for why step family is also stigmatized, that is just social bs like the game argues, there's no logical reason for non biologically related, of age, consenting people to not do it