Birthright: Sister isn’t blood related = Not Incest
Conquest: Sister isn’t blood related = Not Incest
Revelations: Actual blood related cousin = Incest
Extra: Both your non-blood related sister from different families marry each other = Not Incest

I think waste of potential is the most fitting way to describe it possible. There's a good million and six things they didn't do with fates' story that they REALLY should have. The only really interesting thing I can remember is how it just ends up having what just seems like a side character as the final boss in one route.