Sieun's manga / #marriage contract(1)

ch 24 mc gets reincarnated into a novel as an evil sister of the female lead who is abused by her family. mc decides to save her sister and run away to meet the grand duke (who in the novel falls in love with the original female lead). however, things don't go as planned; the supposed grand duke is actually the grandson of the current grand duke and is next in line to inherit the title. obviously mc doesn't want her sister getting married to a grandpa nor the older grandson. in order to ascend the title of a grand duke after ml's grandfather passes away, he needs to get married to a noble girl. and, lo and behold, our mc is a perfect choice. and so they enter a contract marriage...