Why? I'll try to explain it, but I doubt it's something I can make make sense.
It's because we're talking about hypothetical, fantasy, ideal family members, not family members we actually have.
It's like this: You take the hottest girl you can imagine. Okay, now say you live with her with relation X. Somehow, she's attracted to you. The appeal is that your fantasy girl was then yours all along, it's "taboo," you can have it any time you want, you can fantasize about her trying to tempt you, etc.
EX: Let's say... hmm... I don't know, Vida Guerra was my sister. Even if I imagine the circumstances, I can't identify her with actual feelings of sisterhood.
That, and those of us that are capable of thinking realize that there is nothing actually -wrong- with it as long as you don't have children.