NTR no, but just like any other relationship, the distance can strain. Nicole herself wants a mature, successful partner for a relationship. Also the longer the MC stays there, the more likely he is to become attracted to the sister and the others, as well as the sunny atmosphere. Not to mention he'll get invested in the Hotel, and won't be as likely to want to return to the grind of the city.
This will come down to a choice at the end. Stay or Go. Nicole would not be happy living there, she wants her life as a lawyer. So even if you keep your relationship with her strong through the game, you won't get to stay with the Sister after. So you'll end up with "Find a new love on the Beach" or "Return to your current love and being a lawyer"