Scapegoat Sister Vs Second Prince

As the middle prince turned 18, his family was in a hurry to find a worthy candidate who would be beneficial both for soul and origin. My younger sister was invited to the Royal Palace supposing to be a lucky bride, however, that stubborn lady preferred to flee and neglected her obligation! I became the last hope for my family. Moreover, we looked like twins, which could prolong our secret. I was promised to bear a fiance for only one month. So I transform into my sister for 30 days and no more? If the expectation could come true…