It's Difficult for the First-Born Daughter

"My grandmother is cold and my stepmother is still a princess. Although my father and brother protect me, I still have to work hard on my own if I want to live a good life. Then, do you think this is a story about the legitimate daughter's struggle and counterattack? No! No! No! In fact, this is a story that happened in XXX Dynasty, the heartwarming story of a cute girl who is both a doctor and a poisoner. What's going on? Someone is bullying me. You're so stupid that you won't use the poison I gave you. But then, hey, roll up your sleeves and beat him to death!"