Fibonacci's Child is written in five parts. Part One plays out like a 'romantic comedy'. Part Two reads somewhere between 'misery lit' and 'tragedy'. For Part Three it's a visit to the 'S/F' shelf. And Part Four becomes distracted by the 'Fantasy' genre. By Part Five you'll realise the whole series was about mythology and destiny.
Ultimately it's a story about a woman who makes a tragic mistake. The more Katrina tries to fix her error, the worse the consequences become. What is probably unique to this story is the utilisation of a "Groundhog Day" type plot but without the need for time travel. This is achieved by marrying the themes of 'clones' and 'twins'.
Katrina causes the death of her twin sister. When a similar tragedy occurs in the next generation Katrina is quick to recognise a pattern. But twins begatting twin means procreation is always to the power of two. Which twin is destined to kill its sibling is little more that a guessing game. Katrina believes if she can prevent one tragedy she can halt the pattern.
The story is deceptive in almost every way. Whilst the reader and the characters believe they are following a random story the author is simultaneously playing out several stories. "Cane killed Abel", "Zeus split apart the hermaphrodites", "Pollox and Castor are twins but Helen came out of the same egg." there's even a bit of "Heroes" and "Kill Bill" in there somewhere. The story is secretly driven by mathematics, nature, and religion. Katrina thinks her mistake was to clone a twin. Subsequently, the twins want to reproduce 2, 4, 8, 16 . . . Nature wants to follow the Fibonacci Sequence 1,1,2,3,5,8 . . . and is killing them off to keep the numbers right. Katrina is trying to fix the problem using science.