She isn't her sister but just a " good friend ". He will not have choice. He's a simple person without title. He becomes comte thanks to MC. It's the reason why they need MC for the child. So after the divorce he will loses everything and have to pay MC. And the "good friend" loses all, she choose to be a mistress so now she is done. In chapter 19 she even tried to show up because trash husband buys clothes for her. I loves how MC take her down !
There is a novel, but it's only on Kakao. 1 chapter every 3 hours so it's not a quick process for anyone who wants to read. But so far the chapters are 1 to 1 with the manhwa so not missing much yet.