From the back cover:

In a merciless future, mankind mass-produces simulants, living human analogues, to be the slavish workforce of the future. This is the life Mare and Luna were born into: chattel, possessed with a limited lifespan and bereft of personhood.

Joss Breylin, empowered by good grades and a connected father, started out life with far more potential than most humans. He marries the woman of his dreams and they acquire Mare and Luna to help raise the children they hope to have. They adapt when they are unable by allowing the girls to become surrogates, but horrific tragedy...