A lot of countries don't criminalize incest (though it's a recent development) if it's between consenting adults.
Actually, lots of professionals (justice, medicine) refuse to talk about "successful" incestuous relationship and prefer to talk about them, rightly so, under the angle of violence and trauma, for fear of legitimizing such relationships and harming the victims of the non-consenting ones. They do actually exist, but none wants to make a serious study about it, because it touches a taboo.
Without children resulting from such relationships, I don't think it's intrinsically a bad thing (personal opinion) between consenting adults. But if children are the result, it become a whole other can of worms and I can't really condone it.