its just a manhwa..its fiction (⊙…⊙ )
Roselia hasn't gone through poison immunity training. She literally isn't in the position for it. Ein has gone through it and was able to protect her. It's obvious that current Roselia is very unaware of the things going on in this world. She isn't gonna be actively on the lookout for poisoned drinks. Like the user above said its just a manhwa its literally for plot and character development.
i'm sorry but nah, you're forgetting about isekai'd people at this point. hello? most reincarnated people started as a child and they plot ad protect the older people
This is where I draw the line. In what world should the child be protecting the parent like maybe when they’re older but def not at such a young age. Lol I’m mad cuz ur parents should be able to handle the truth and love you especially because both the duke and Ein are placed in that position by the emperor and to survive.