in Japan people can mary their cousins it is legal there
It is not always considered incest. You should educate yourself on that, and go look that up.. In many cultures it occurs, because - shit happens! It is also legal in some US states to marry your cousin, you just have to check each State's laws. They're all different and sometimes classified due to different situations. However it is not recommended due to the stigma it carries as well as the myths in regards to they may have children and the gene pool is not diverse in some cases and that the kids could have certain issues due to that. But you can always get a test before having any child with any one (not just cousins) to make sure that your gene pool is diverse enough and will not cause problems.
Like seriously, they are only four years apart. It's not that big....
In some states, you can marry your cousin only if he or she is a 2nd or 3rd cousin. The farther apart are you in relations, you are less likely to have genetic problems
The guy sensei is all like "our age difference is big" but i think he misses the main problem that he is the cousin of the girl he likes!!! And no one seems to bother about it in the story too. I'm not against incest, actually i like it but notice it please. A comment about the incest problem will do.