According to the book (so far I have read around half), Curt and Honora are very much in love, together. The ONS was not even a ONS - it was a few seconds of lust (but in the book they did go all the way). But the main difference from what they have shown to date is that the Pia character (Crystal in the book) HATES Curt. She seduced him out of anger because she is angry that he took her sisters away from her. So Pia's turning negative is more or less in line with the book. And unfortunately in the book, Crystal does have a child, while Honora is unable to have a child.
The slight difference is that the Rano character (the younger sister) is actually probably Rashi in the series - we've seen the beginnings of it in the episode where Bani tells folks that Rashi was abused.
But where the book and the series divide is that so far, the Curt/Honora (Jai/Bani) couple are very much together facing everything together. Haven't reached the part where she finds out about the lust thing, but they are the central figures in the book and all the other couples revolve around them. Why Ekta is putting us through so much pain when her inspiration doesn't is beyond me.