Responses
He was legit trying to help his sister with the MC (also to get some revenge for his daughter). They plotted together, but he was not going down with that ship. It also doesn't matter that he used her. She has 0 options left. She has no life skills, so she will have to depend on her brother to survive.

Did I misunderstand something. I though Concubine Xiao (the one with Brand) concocted allergy plot to get MC using info she got from her brother, the general. Using little princess as convenient victim to get to MC. But it kind of looks like her brother, the general is the mastermind who actually targeted Emperor's daughter using his stupid sister crusade against MC to shift blame from himself to MC? With MC (and his sister) being scapegoats for his actual plan?