I don't think tsunderes push you in front of moving cars or slam glass doors in your face so your arms get cut open by the sharp glass and then pretending that it wasn't her fault *sigh*.
Key element of tsuderes is that they never get dangerously violent while my sister has done so a lot of times. Like when she was flipping pancakes she decided to rid of the oil on the spatula by waving it around while I was there pouring my drink. 200 degrees C oil droplets hurt man.
Only one thing comes to mind:
Well calculated and sever revenge upon such an individual.

On to the topic at hand...
The fact that close blood relationship inbreeding, "incest" has a large chance or probability of causing anatomical and physiological problems has been a long studied and proven fact. Aside from from that, even if somehow the offspring isn't diseased or deformed, there is also a strong chance of psychological trauma, from either the child discovering that the parents are close blood relation and reacting from sociocultural ingrained thought or it being found out by others that the child is begotten from an incestuous relationship or even both might occur, which has a strong likelihood of psychologically scarring and/or making the child dysfunctional. There is a greater threat to inbred offspring then just the bad genetics that are highly possible.
Now on to the matter of intelligence. It seems to be forgotten that whom and what we are is a balance between 2 things; Genetics AND upbringing or environment. Statistically speaking, 2 intelligent or well educated, "smart" people have a much higher chance of producing smart children then "stupid" people, which is simple logic and statistics. Studies have shown that 2 smart and/or educated people will raise their offspring in a similar educational environment as their own, as will 2 "dumb" people. There you have the balance between genetics and upbringing; good genes plus good upbringing will produce good children, while bad genes with bad upbringing will produce bad children. Of course there are exceptions to the rule and I am only speaking in general terms, but those are the accepted norms.
As for "incest = retarded children", well yes, statistically there is a higher chance of retardation from a variety of inbred problems ranging from development to metabolism, but I
personally don't think that any couple that would produce children from an incestuous relationship would be all too well educated, which goes back to my statement about breeding and upbringing. Incest, performed by most likely badly or un-educated people has a higher probability of not only have bad gene configurations, but also then raising their offspring with a poor educational background.
If a close blood relation couple is in love and want to be together, I really don't care, but if it produces offspring that will even further contaminate the gene pool, to that I ardently object.