Honestly I don't really remember that, but I think "an heir" is what was supposed to happen. If I'm not mistaken in her past life, ML had a son with another woman after the divorce? So maybe that was where the mentions of a "son" came fromm? Idk honestly it's time for a re reading for me.
But might I ask what's wrong with her having a daughter? Even if the laws are against them, after becoming dukes they can just change them.
No there was never a son to begin with. In past life too there was no successor of arno (girl/boy) . In novel it is mentioned that Alec was the only successor of Arno and a girl, it's just that she is a tomboy and even became a war hero in future. She will marry the Prince of empire and take over the throne. The author clearly mentioned Alec being a girl (tomboy and muscular)
Bruh wasn't Bianca supposed to have a son and the author even hinted that many times but suddenly they just decided scrap that and then changed it to a daughter instead ( ̄へ ̄)