As far as I'm concerned it needs to be between consenting adults who can agree to what they're doing, who are mentally fit and mature enough to understand the consequences and are in charge of their own lives. No forced marriage, underage kids, animals, that sort of thing, because one or more of the parties really doesn't have a say in or understand what's happening.

As for family members, well, that may have been a common thing for many centuries, especially in royal families, but I don't think it should be allowed now simply for the genetic problems it creates in offspring: physical and mental deformities, insanity, etc. I don't think any child should be subjected to that kind of miserable life, and it would be of no benefit to society or the gene pool. Maybe if the marrying parties were sterilized I couldn't logically object to it, other than it just seems weird...I sure as hell wouldn't want to marry my brother after having to grow up with him! =p