I don't think they fed him poison, I think Lister probably does think a sorta version of what he said before. He's just a nasty idiot. I'm assuming the sister knew a lot more than he did, tho could be wrong.
Have not read the novel so I'm just theorizing, but probably what happened is something along the lines of, the Glenbergs were told to help the other side with something in exchange for more power, but they had not realized just how fucked over Sylhan would be because of it. Then they got in too deep and could not take it back let's say. It would make sense then why the sister always acted so guilty towards Sylhan and tried her best to protect him and hoped his status would be restored, whereas the brother was dismissive of the whole thing and just kinda happy he is more powerful than him. I feel like if they had actively chosen to betray him their attitudes would have been different.
Btw knights can be nobility, in fact most high ranking knights are from noble families, so I don't think that them being nobles is a new upgrade from being knights, their estate afterall can't be new.
I did read it i I am confused as he'll so I see he got his memories and originality lister and that botch were royal guards so did they feed him medicine than manipulated that he did something so wrong and they were the one saving him even and why was the f bitch was a king did they perform a coup or what and what are the fking betrayal and mainly how they become royal if they were knight