That is a very good question.... But until now this artist didn't NTR so i don't think it will be the case. Even for the works of the gyaru girls the sex with others was before meeting the MC of the first work and thus were but flashbacks in the second one so. Also by the look of it, it ain't going to be the case, we might get a sequel with the sister tho who knows. Also i expected the MC to be the guy that get bullied at the start but i was wrong xD
Not sure there's gonna be an NTR sequel. This is vanilla through and through. They are cousins tho, so take that as you will.
The MC was "reformed" halfway so he dyed his hair colour back to black. I guess it's more like his uncle/aunt's demand but well, I can see why people got confused. The bullied guy did make appearance later when the girl introduced herself as the MC's wife, when he was asking the MC to be his martial arts teacher.