No. That’s just incorrect. The hentai and this storyline are taking place in the same time. And she’s never his lover or claimed to be his lover - she’s always been his big sister.
All these stories share a common theme. The MC finds someone that makes life worth living for. In this case it’s fmc. Then what about someone who has no one? It’a a fictional story of course so that person won’t have someone magically barge into their life, go on an adventure, and have a life...