The story has a few flips. With Mayuki Ito teaching his brother how to have sex with this girlfriend and giving it a practical try.
Then when she was getting jealous of him & his GF. Then Mayuki making him jealous back… In the end, the power of inbreeding can’t be stopped.
About Mayuki Ito, I have never seen a woman lose charms faster than her. Admittedly, she is still pretty and has a substantial hot body, but still. Somewhat reminds me of Asuka Kirara.