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Now I have to go back and read it again. Was a childhood fave, which means it has been a few decades. I enjoyed the Phoenix taking David on a tour of classical mythology.
Welcome! I think you can improve your question with a little reorganizing - honestly, I am a little confused - and some additional clues. For example, when did the series come out? When and where did you read it? Did you read only the last book or do you have information from the other books? Like what the other superpowers are and how the girls acquired them? If you don't want to reveal the ending, you can use the feature that will mask any spoilers.
Welcome! Can you recall how her having cancer fit in the plot? What country did the story occur? What happened after she discovered that the children were clones? Any details might help. Sounds intriguing.
About that time, a publisher might prefer that authors whose proposed books were over a certain length, determined in part with the limitations of the presses used by the printer, would divide the content into two or more volumes. Series make more money than a single book. I used to work with mainstream authors; they were pressured to sign multi-volumes contracts - sometimes for one book "chopped up" and sometimes a series, perhaps with the same characters. I know one successful author who refused to sign such a contract - made a lot less money, but less deadline headaches.
This is a wild guess. Sometimes made-up words that start with "Z" are inspired by Greek words that start with "X." So looking up Xene one gets: French -xène, from Greek -xenos stranger, from xenos.
This has a Poul Anderson flavor. He wrote stories that mixed historical references to ancient and mythical civilizations with time-traveling modern people. However, I don't recognize the plot.
