I don't think there is a single character in this book who has ever made a rational decision. Anti-heroes and flawed characters can be great. Sometimes it's fun to hate the protagonist a little bit. But there was nothing fun about Rhonda. She was just a desperately needy, vapid drama queen.
The "big twist" was completely predictable. Stupid decisions were justified as being done out of love, but they were in fact just acts of cowardice and selfishness. The big lie about Olivia's parentage, for example. How was that in any way an intelligent or loving thing to do? What was the point? Not to spare Olivia heartache, but to allow Rhonda to take advantage of her sister's fragile emotional state to satisfy her own desire for a child.
Above all, I become extremely frustrated when women's fiction presents female characters who have no sense of individualism or inner strength, and exist entirely for other people. What's worse is when they don't seem to outgrow this by the end of the story, which was the case here.