Imoto to Danna Sama Ni Kodomo Ga Dekitanode, Rien Shite Ringoku Ni Totsugimasu (Regina Books) [Light Novel]

[Machine Translation] Elesia is the eldest daughter of a marquis and has been disciplined strictly. Thanks to her discipline, she was discovered by the duke and lived a happy life with her husband Farman, even though they were unable to have children. One day, however, she discovers that Farman is having an affair. To her surprise, the man she was having an affair with was her sister, Lena, who was pregnant with Farman's child! Elesia is disappointed to find herself divorced and returns to her parents' house. One day, Prince Leon of Artsburg comes to visit her and proposes to her, saying that he has come to fulfill a promise he made 13 years ago. Elesia accepts her father's "certain conditions" and decides to marry Leon, but when Rena hears about it, she begins to show some disturbing behavior...

エレシアは侯爵家の長女として厳しく躾けられてきた。その甲斐あって公爵家に見初められ、子供はできないながらも夫ファルマンと幸せな生活をおくっていた。だが、ある日ファルマンの浮気が発覚する。なんと浮気相手は妹のレナで、しかもファルマンとの子供を妊娠していた!?離縁することになり、失意のなか実家に出戻ったエレシア。両親から嫌味を言われ鬱々と過ごしていたが、ある日隣国アーツブルグの王子レオンが訪ねてきて、「13年前の約束を果たしに来た」とプロポーズされる。父親が出した『とある条件』を呑むことでエレシアはレオンに嫁ぐことにするが、それを聞いたレナが何やら不穏な動きを見せていて―