In the past of the empire, the aristocracy had significantly more power, and they commonly abused the commoners. However, a silver-haired man with heterochromia (multicolored-eyes) basically destroyed that, so there's a stigma against those with one or more of those traits if born in nobility.
Shirley was born a Duke's daughter, but was hated because of how she looked. So, her younger sister tormented her and abused her. Crying, she was seen by the prince, who was charmed by her and asked his father to make her his fiance. (The royalty liked the hero of the past because he allowed them to regain some control from the aristocracy, and they also liked Shirley.)
As the years passed, Shirley's sister slowly poisoned the prince's mind and slept with a LOT of important people. On the day of the wedding, the Prince broke the engagement and had his faction arrest Shirley for fabricated crimes of abuse against her sister (which obviously wasn't true).
(The Emperor, Empress, and Princess were unfortunately away at this time.)
Shirley (pregnant) was tortured to force an admission, but she refused to break. She eventually escaped and swore to get revenge, training harshly and amassing power.
When the Emperor, Empress, and Princess (who all treated Shirley lik actual family) returned, they were (obviously) quite pissed, but the Prince's faction assassinated the former 2 (the Emperor and Empress).
The Prince became the emperor with MC's slutty b(tch of a sister as empress.
(Princess survived and is quietly weakening her brother's reign from within...)
Shirley awakened her semi-immortality and gained all of the power and skill she has now through severe swordsmanship and magic training, but giving birth to twins forced her to put aside her grudges for the time being...
Ironically, the new empress was such a slut that she got STDs and ended up infertile.
After years of believing that Shirley was dead, they find out that she is not only alive, but an extremely famous adventurer with TWO daughters.
In order to secures heirs as well as the empress's position, Emperor Scumbag plots to get "his" daughters back and adopt them into the lineage.
Assuming that this manga isn't dead and still receiving updates, Emperor Scumbag and the Empire should re-enter the plot in 1-2 short arcs from now.