"Isn't it fine to just give birth again if he dies"
I guess that's demon logic for you. Maybe demon gestation and growth to adulthood happens all in less than a year or something, to say nothing about lacking the emotional investment and a little something called love that makes many humans value their children. Lunatica's name is pretty fitting.
I get the feeling the entire thing was a setup, maybe by Sakurako "preparing the stage" for whatever it is she wants to watch from her former brother. I'm surprised both Yui and Silkie played hero in this incident; I too don't understand why Silkie didn't take the chance to attack Yui. Was she waiting to see her use her cheat skill to better understand her target? And if it was Sakurako who told Lunatica to stop, it bolsters my idea that she's manipulating things to create a stage for Silkie to show off in, and Lunatica doing her job ruins that.
Anyway, instead of Yui showing off her cheat skill (which is probably happening next chapter) it's Chika and the adventurer girls who got to shine this chapter. But I'm still not convinced they'll be alive for the long run. They look strong now, but it's because they're against nameless monsters and not named characters.