An Incompetent Woman Wants to Be a Villainess ~The Young Lady Who Married as a Substitute for Her Stepsister Didn't Notice the Duke's Doting~ is a Manga/Manhwa/Manhua in english language, ROMANCE series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

Avril is a young woman of exceptional ability who was mistreated and labeled "incompetent" by her family. Avril marries into the family of an "old, lustful duke" in place of her half-sister, but the duke, Dylan, is actually a beautiful young man who is the complete opposite of what he is rumored to be. He wants to marry a "villainess," and a contractual divorce her after three years is perfect for Avril's future freedom. The people around her are bewildered by the mysterious "brilliant woman" who plays the role of a "villainess" with gusto, but they are gradually drawn to her..