I'm not going to make it but this is how Nicole is the MCs mom. She had him when she was very young and gave him up for adoption without letting John know (he was away at college?). Then John comes back and marries her and they have Chloe so Chloe is the MC's sister. Nicole doesn't want the MC dating her daughter because she thinks the MC could be her son but she isn't sure. John's actions stem from him thinking the MC is a lot like himself when he was that age, not knowing he is the MC's father.
The MC fucks Chloe without knowing he is fucking his sister. You can throw into it the Melissa is John's daughter from a blackmail thing if you want.
The aunt could be dying from a genetic disorder that eventually the MC's parents will need to come clean about and reveal that the MC is adopted leading to the MC searching for his birth mother, not knowing that he has been fucking her and his sister for months.
Of course this is just idle speculation.
The MC fucks Chloe without knowing he is fucking his sister. You can throw into it the Melissa is John's daughter from a blackmail thing if you want.
The aunt could be dying from a genetic disorder that eventually the MC's parents will need to come clean about and reveal that the MC is adopted leading to the MC searching for his birth mother, not knowing that he has been fucking her and his sister for months.
Of course this is just idle speculation.