So, you think it should be legalised then? Or are there other arguments against it? What about the conflict of interests in terms of relationships? Where does caring for a relative turn into sexual interest, and is that a problem? Also...do you not think it would make abuse more likely?
I think it should be decriminalised, which is slightly different from legalised, I don't think two consenting adults in full possession of their faculties should be made in to criminals for doing things which are none of my business or your business or anyone else's business.
I don't know of any relationship in which conflicts of interest don't exist in one form or another, the law has always been subtle enough to cope with them thus far, I'm confident that we have the wit and wisdom to be able to cope with what would in any case only be a very very tiny amount of instances of these kinds of relationship.
I don't presume to know the vagaries of the human heart, so I can't speculate about any of the simply multitudinous reasons why people may be in relationships with one another, it's none of my business if two people love each other because they're co-dependent or not nor is it my business to speculate about the health of that relationship unless it directly effects me, and logically speaking neither is it any of society's business. This applies to an incestuous relationship as much as every other kind of relationship between consenting adults.
Erm you question about abuse is an enormous one. We have currently absolutely no idea about the true prevalence of a variety of kinds of sexual, mental, and physical abuse. The figures we do have represent reported cases, which may represent either the tip of a gigantic iceberg or a percentage from which it might be possible to extrapolate, frankly we just don't know.
I don't think it would be fair to blame adult incestuous couples for peadophilia any more than I think it would be fair to blame adult Sado-Masochists for spousal abuse.
One of the old arguments against legalising Homosexuality used to be based on a similar confusion, i.e. that because Gay men are attracted to other men legalising it would encourage greater levels of abuse of male children. The problem with that is that it confuses superficially similar details and then presumes outcomes from them without realising that the the details which have been confused are in fact crucial to understanding what is being discussed.
Legalising homosexuality means legalising sex between adult men, not sex between adult men and male children, and in fact the two are fundamentally different.
In reverse a similar argument used to be used by extreme feminists against marriage, that by marriage being a legal institution which traditionally had seen women as subservient to men (the whole "love, honour, obey" thing) that marriage itself encouraged men to see women as objects, therefore increasing the instance of rape.
What that argument failed to realise was that rapists are normally pathological, as are paedophiles (paedophilia being strictly speaking, and legally speaking, a form of rape) and that instance of both rape and paedophilia will probably always only be as prevalent as the pathology which causes them is.
I don't think decriminalising adult incest will be any more likely to cause more instances of incestuous child abuse than legalising homosexuality is to increase instances of men abusing boys, because the pathology which causes child abuse of any kind has nothing to do with the feelings and attractions which two adults have for one another.