Sayonara, Morashita Otto.: Saretsuma Futago no Irekawari Fukushuu

Pregnant and stuck at home with her emotionally abusive husband and self-serving in-laws, Sekka tried to grin and bear it by holding onto the hope that he would change once the baby came. Then she finds out he's been cheating...and while she's still reeling from that news, he deliberately sets her beloved pet cockatiel loose, which finally pushes her past the breaking point. Unable to stand by and watch her suffer anymore, her twin sister Rikka makes an offer: "I'll take your place and get revenge on your husband!" (Source: Manga Plaza)