To be more specific: What is wrong about a romantic and/or sexual relationship between siblings?

Arguments I've found on my own and my responses:
1) If offspring results, it'll probably have some severe genetic problems. Big deal, don't have kids.

2) It's illegal in most places/Many people will shun you if they know about it. While these are valid practical considerations, I'm more thinking "wrong" as in morally/ethically. These are symptoms of wrongness, not the reasons.

3) It's unnatural. That's what they said about gays too. And not true. Get a male and a female rabbit and watch their offspring.

4) It's weird. What isn't?

5) My Holy Book says it's a no-no. I don't believe in it. Also, see 2). This is another symptom.

6) It will adversely affect the siblings' psychology, due to already established standard sibling relationships. Now this one, if true, could be a good explanation. Anyone here who did some research into this?

And before the jokes start: No, I'm not an inbred hick. I'm just wondering about it. Had an odd dream. No, not of the wet variety.