The father seems like a good man so maybe the mother of the first son died and the dad remarried to the current mom and had Robel then the daughter. Hope that makes sense and this is just a guess
well what do you expect she is a DEMON KING destruction is par for the course in her line of work so ya it is kind of to be expected that it would not be an issue if she casually says hey i am gonna go wipe out a village or 2 lol
anyone knows where the raws are?