No idea why Japan still is going on with this cousin incest theme.
To be honest it's shocking to know that children between first cousins are losing around 10 IQ points, which is a lot even for East Asians (average about 110/120 IQ, depending on which system/scale is used), and are on top of that more prone to have hereditary genetic defects, so a cousin couple would have to abstain from passing down their genes (though having one of them passing on his/her genes would be possible with artificial insemination or surrogacy).
Aside of these options, of course adoption could be an option, too, but if the mother didn't bear the child, she has way less connectivity with the child, so problems could be arising everywhere if you don't forgo your natural wish for a child...
I know these manga are usually just about family life and romance, but they need to give the audience a realistic perspective on life,too, if they aren't some type of science fiction or fantasy story.