if it's between consenting adults, it should be fine (and acceptable for marriage), according to this forum and many Deomocratic leaders.
Sure, why not? If it's between consenting adults, why not? As long as they're not hurting anyone else, where's the harm? I mean, some of it is weird... like incest. Yikes. But hey, whatever floats your boat.
The only one I might have a problem with is incest between brother and sister, as that yields a very, very high chance of birth defects. (Note: This is not to say that I would have sex with my brother, even if I had a broither. I would not. Ick.)