Get lost shitty prince. You just want her, because you suddenly can't have her. You also fell in love with the younger sister, because you wanted to protect her. Those are not bad bad reasons to fall in love with someone, but in your case that love is shallow. Your treatment of younger sister this time around and your behavior towards MC last time and in the beginning of this time, shows this clearly.
Yandere-Yulan is a much better pick.