Bruh, take a look at the times.. Shuli was (definitely) going to be sold by her family eventually to god knows what pervert. At least with Johannes, she experienced life as a noble, gaining knowledge she never would have as a commoner. Essentially living in the lap of luxury. Despite not being well received, it's still a quite a fulfilling life, and at the end will have the chance to remarry.
Agreed, it was a stupid and reckless idea and basically child abuse (hence why it failed so hard the first time around). Plus, looking at the portrait of the previous empress, we can all see where this braindead idea came from.
BUT the story is fascinating and I wanna see more of Shuli's transformation into a bosslady despite the struggle!
If he adopted her then she wouldn't have automatically become the head of the house. Even if she was older than Jeremy as his adopted sister, Jeremy would still be the one to become the head of house because he was a male. But Johannes already deemed Jeremy not worthy at the age yet. But by marrying Shuli and making her Jeremy's step mother, Shuli could watch over his family while also protecting herself from the fate that her real family was going to push onto her. It was a beneficial relationship for the two of them that Shuli had ultimately agreed to.

Honestly, I hate the siblings' father... He basically bought shuli (who was a poor child), married her (knowing that she wouldn't be well received) and made her go through all kinds of training just so she would take care of all the difficult things (managing their family, etc) after his passing while his older son could freely act like a child his age, free of responsibility. And then after Jeremy becomes an adult shuli would hand everything to him and step down. He literally sacrificed shuli's childhood to preserve Jeremy's... Pretty fucked up if you'd ask me. I know shuli likes him but I can't perceive him as a good person at all