Just a quick correction on your original post, the relation between Kevin and his sister does not qualify as netorare/netori. Both of those implies the lack of consent or knowledge from the partner that is beeing cheated on (Kevin here).
The relation as you describe it (haven't played the first game in year, and probably won't play the follow up if what you says is rigth) would be simply cuckoldry, or netorase (third variant of NTR, poorly know cause rarely used in porn, and generaly just called cuckoldry)