Unborn queens kill their motherUnborn queens do not appear to be birthed naturally and instead need to be cut out and removed, as happens with Diva. This might explain what happened to her and Saya's mother, who died pregnant in a mysterious condition due to unknown causes (she was mummified, not crystallized, meaning another queen didn't poison her with her blood even though this is explicitly the only thing that can kill them, and she was in the shape of a bestial chiropteran, not the human-looking queens who appear later without showing any monster forms). As their mother didn't have her babies removed, it's possible that she died from them continuing to put a drain on her body's natural resources longer than they were supposed to, ultimately leaving her so weak that her chiropteran body could no longer sustain its human form (or possibly the overload of chiropteran hormones in her body transformed her), devolved into her base chiropteran form, and eventually died.
What happened to the previous generation(s) of chiropteran queens...With the examples we have in the series, it's implied that queens are always born as twins, so in theory at least one previous generation's queen (eg their aunt, grandmother, great-grandmother) should be out there somewhere. Taking into consideration not only the queens' apparent instinctive animosity towards one another and the current generation's personal reasons for wanting to see them gone, there are a couple possibilities:
- The previous queens already killed each other off and any survivors were subjected to some Fate Worse than Death at the hands of a rival queen's surviving chevaliers (like Nathan) in revenge for their dead queen. Chevaliers might be able to get the upper-hand even on a well-fed queen if they are able to mass-attack her, and from there they might be able to trap her and starve her to make her weaker.
- The previous queens were Killed Offscreen by Saya, who is on a mission to exterminate the chiropteran race entirely, actively hunting down chiropterans, and also has little reason to trust another relative after meeting Diva.
- The previous queens were Killed Offscreen by Diva, a sadistic, kill-happy sociopath who would also have little reason to trust them, would kill them to eliminate the chance they could kill her or her daughters in the future, and to eliminate competition over food sources and rule the chiropterans herself ... oh, and her daughters).
- The previous queens are in hiding. They're older and wiser than Saya and Diva, so they're used to the traditional covering their tracks and keeping everything on the down-low. Everything going on with Saya and Diva exposing all their secrets and airing their dirty laundry for the world to see through chiropteran mutations and experiments, and entire human organizations who know about them, would freak them out if they knew... which they might, which might be why they chose to hide.
- The previous queens killed their only source of death, their respective twins. The queens offsprings wouldn't be able to harm the queen, so there would be no reason to kill them. Chiropterans have existed for years outside of human knowledge, and it's only when humans used them that they were mass produced for the destruction of mankind.