I think it's a that and the dad being nobility. Nobles tend to get slaps on the wrist (compared to normies). The step mom and step sister don't have that protection or authority.
I'm not just talking about the legal punishment, but also the author's mindset regarding parental neglect. FL herself thinks she should understand her father better, as if what he did were justified and as if a child should rationalize why they are neglected/abused. The author gives the impression that he is just a poor guy, and that's the problem.
oh! yeah that way of looking at things is pretty common. Evil step moms and idiotic/doormat/negligent/absent dads we should automatically feel sorry for have been a trope since forever.
Honestly, the punishment of FL's family couldn't be more disappointing. The author acts as if the stepmother were the ultimate villain, but the real villain in this story is FL's father. He abandoned his wife and left her to die alone, abandoned his daughter, and blamed her for her mother's death, which is his responsibility. Less than a year after the death of the woman he "loved," he already had a new daughter and wife. And worst of all, he spent 23 years torturing his daughter and then sold her, knowing she would have a miserable life. But in the end, in the author's eyes, he's just a poor man whose only sin is being a coward and working too hard? So hard-working that he didn't even notice what was happening in his own territory? And so the biggest villain ended up without punishment and without reflection on his own actions, because a man never suffers the consequences of his own actions. It's truly ironic not to know how to interpret the work you yourself created; your ability to interpret reality is less than that of a door.