The MC is a girl who was adopted and supposed to marry the supposed male lead in place of the original daughter. In the original timeline she did and became the Empress. The original daughter was reborn and decided to return home as running away caused to live a bad life. The MC was then forced to marry a dying man, the former Crown prince and son of the previous emperor. The original daughter got her wish and married the male lead. The MC rescued the ML and nursed him back to health. The ML is supposed to have superhuman strength. He was sick because of slow poisoning over a period of time. I just can't remember the name of this novel. I've tried all kinds of searches but still can't find it.
I’m not sure if it’s this one but it’s a similar one, Transmigrating into the Female Supporting Character with a Good Life in a Laid-Back Novel