Ivor, on feelings vs rationality.
If it is a matter of feelings, then yes, in many cases feelings should predominate in deciding the issue.
For example: a parent continually berating a child is generally known to cause all kinds of emotional issues (I'm not going to cite or debate that - feel free to change the scenario slightly if this one doesn't sit well). In such a case feelings/emotions should be the overriding decision maker, yes? We are, after all, thinking and feeling animals; denying half of that is rather short sighted.
I suspect some will find the above not rigorous. After all, I am talking about the feeling of the child, but the OP asks about the feelings of the decider, which in this case certainly is two different people. While I concur with that, this scenario is just a quick example to make the point that feelings matter. I'm trying to write a forum post, not a book on ethics. Alternatively, I suppose one could argue that I'm making a rational decision, in that I'm using reason to decide emotions are important. I'd consider that a cheap trick - parsing a question and answering it requires reason, so that line of argument would automatically invalidate the 'feeling' answer. I'm not trying to put words in anyone's mouth, just anticipate possible objections. And, obviously, you must strike a balance. If I "feel" like I should punch you in the face (I don't) - I would hope it's clear I'm not advocating that I can just start swinging. We need to balance everyone's well being and rights, and to do that we need reason. IOW, favoring just one side of that equation to the exclusion of the other is when you end up with messed up situations.
In the case of the OP scenario, although perhaps those specific 2 people are not harmed, I suspect research would bear out (I'm not going to spend my day googling incest, thank you) that beside the birth defects issue there is probably a lot of risk of various emotional trauma, power imbalances, etc. I would guess it is not an easy topic to ethically research. How do you tease out emotional damage due to violating a taboo vs emotional damage caused even if there wasn't a taboo? I dunno. I suggest, strongly, putting a priority on our feelings about a situation over being 'rational' when emotions play a factor in harm.
edit: I guess to summarize this I would say that our feelings about an issue are often rooted in how we would feeling about an instance happening to us. I react in horror at the thought of somebody killing me for no reason - I don't need rationality in any great amounts to conclude murder is off the table. OTOH, it isn't that hard to construct functionally identical scenerios where a person will choose opposite actions just based on the wording of the problem. I'm thinking of the famous train problems where you switch a train to travel on track A or B and end up killing or not killing people. In those cases our feelings and intuitions seem to be unreliable. Blindly reacting to emotions is not the answer, but neither is purely relying on reason. Are emotions are based on our biology, and run deep. That they lead us astray sometimes is not any reason to ignore them, just to view them with suspicion.
