ML is Clyde, they married at the end. Clyde's the duke illegitimate son that's why he was sold as a slave at some point in the past. But he was sold again to the imperial family for his swordmanship talent.
Stella has an older brother Joseph and an older sister Karen. The family of the emperor is supposed to have special abilities related to the sun(Joseph), moon(Karen) and stars(Stella). The extent of their abilities were note properly explained, But they can hear them (stars, moon, sun) and the MC can for example remove the stars from the sky or fill the sky with stars, however they dont have any real power as in power to do sth useful. What the sister and brother could do was not properly explained. Their abilities are purely used as proof that the heavens support them, and it simply keeps the people in the empire believing in them.
Now Stella not able to display her power when she was a child was bad news for him, bc it will weaken the faith of the ppl. Thus he spread a rumor that they are hiding the precious youngest princess away because she was too sacred to be revealed to the world. But in reality he disguised her as the maid of Karen.
And when a rebellion happened, they flew away lefting her there expecting her to gain some time for them.
10 years after the now Duke Clyde found a woman who ressemble her a lot and got interested. Karen learnt about it and bring her to the palace, asking her to act like the real princess but in reality being a fake (she didn't knew it was the real one).
EDIT : I read some comments around ch75. "She should just join forces with Clyde and kill off the entire royal family and tell the masses the stars ordered her to do it or the empire will fall into ruin. Rule herself while bringing her adoptive family into the palace and finish this boring story that’s needlessly dragging. In 70+ chapters all she’s had in revenge is having her sister jailed."
An Extra In The Family Is The First To Be Abandoned