NSFW Why is incest so bad?

Aside from the obvious, which is defective DNA and genetic coding.

Also the question does not refer to sexual abuse of a minor.

What lead me to ask this question was watching a Documentary of siblings being reunited in adulthood, after years of separation and falling in love. The similarities in characteristics could be a major attraction factor.

I knew of two sisters who identified themselves as gay, that being separate to the fact that they as adults were having an incestuous affair. What is wrong with that, in your opinion? Do you know of any such cases personally and how did it pan out for the couples involved?