If they don't have any children, there aren't really any good reasons anyone can bring up to why it's wrong. The best one can say is something along the lines of it "feeling wrong," or a view which would exclude all non-offspring bearing couples but there are a lot more issues that occur if someone takes the ladder stance with incest being the least of such worries. If there is a large age gap between them then a discussion can be had for those cases but that wouldn't be in relation to the incest itself but the age gap, then for example in the case of two twin sisters/brothers falling in love it's quite hard to logically refute.