Well it
is
Well it is up to you, but basically the sister is the relation for MC setup from the very start of the game and that is shown in game even long before he even meets Nicole in game, not that so called GF you are so hung up on. Something happens in family situation and they get separated and MC makes the mistake to start a relationship that is not good for him and his new GF. Then gets a call from his sister to come help her. Basically that is the start of the game. Nicole is officially his GF when that calls come in, but she is not the one the game meant for him.

Now if you do not like that or the other option devs offered since some did like Nicole, this is not the game for you.
It is about the growth of an MC from his past that through reconnecting with his sister finds he has other opportunities as just the one his father tried to force on him and that his current Ex-GF is also a part of. It is also about possible relations, sex, old wounds and new acquaintances of course in a paradise setting.