Greg responds...

Well, the immediate inspiration for the Sisters was obviously Shakespeare's Weird Sisters from MACBETH. But a surprising number of cultures have myths of a triple goddess, generally associated with fate and/or the phases of the moon. As I recall, Seline (or Selene) was the name of the Greek goddess of the moon. Luna was a roman name occasionally used for the same goddess.
Phoebe means "bright" (in its feminine form). It was a epithet occasionally used almost as an adjective to describe Artemis (or Diana), the goddess of the hunt, who was also often associated with the moon, just as her twin brother Phoebus Apollo was often associated or confused with Phoebus Helios the god of the Sun. In fact, I had a wealth of names to chose from.
So I finally just picked three that seemed to work well together.
I had many plans for the clones and the Sisters. Too much to relate in this Q&A format.