Just my two cents:
The mother is not "in on it." She wants to see her daughter accept her step-father as a dad and become a normal family (don't all single parents wish for this when they remarry?). She is there only because the story, quite literally, could not happen without her (notice that the mother has almost no voice in the story)!
The mother's infertility is there to imply that the daughter can give her step father something (i.e., a child) that her mother cannot, which suggests that the daughter sees herself as not just a "third wheel" in her parents' marriage (this also answers a moral question as to whether the daughter's actions are "wrong," since she can point to the above and argue that the end justifies the means). In this case, making a child is the same as making a family.
The ONLY reason the mother's character stands out is because she is the only woman who is not portrayed as a potential target of her daughter's yandere disposition. However, I strongly suspect this will not remain true. I have no evidence from the story to support this suspicion, but it is typical of a yandere character to become more unhinged as the story progresses. So it is hard to believe that the daughter will remain content with the status-quo, as her step father continues to reject her and seek intimacy exclusively from her mother.
Lastly, the word "family" is likely overused in order to emphasize its myriad connotations. Chapters 13 and 14 both demonstrate that different characters (e.g., the step father's mother and sister) hold competing ideas of what constitutes a legitimate family. So I caution readers not to assume that just because two characters use the same word, that they are describing the same thing (particularly when this is used to argue that the mother and daughter are of one mind).