Cinderella no Ane Desu ga, Fuhoni Nagara Ouji to Kekkon suru Koto ni Narimashita

After Cinderella rejects the crown prince's proposal and elopes with a magician, her older sister Adelina gets forced to take her place. Although the popular and handsome prince marries Adelina, he tells her he does not intend on falling in love with her. Adelina could care less, however. To her, all that matters is that she be able to enjoy living quietly with her adorable animals at the royal villa, eventually getting an amicable divorce and a ton of alimony. Who cares if the prince gets a mistress or two? But the prince, who didn't care for Adelina at all, begins to show some interest in her...

(Source: MangaPlaza)