MC said it best himself. She's already 18, doesn't want or need anyone and already has her own life, what's she need him for at that point? Would have made more sense if she simply wasn't 18 yet and her grandma was proposing something better than foster care after her death for the next 3 years or something if she was 15 for example and simply not an adult yet. I mean f/d incest is illegal in most places anyway so wouldn't be much difference if she was also jailbait too, just another additional reason to keep the relationship secret when it got that far.