TaetaeCollarbone

What sort of a reaction is that

Even in thr previous timeline, the crown prince didn't really like the bitch sister.
After their first encounter, the protagonist thinks about how he probably got married with the sister for political reasons in the first life.
My theory is: the prince didn't want to get married to a brat with an awful reputation who bullied her sister (even though she was manipulated to do so), but still wants to have a connection with the Ducal house. The Duke favours the illegitimate daughter and didn't care about Claudia, and the brother despises her for being a spoiled mean girl, so they probably had already decided to kick her to a monastery, specially since she was just a candidate and not a fiance. The prince just acted accordingly: if he gets married to the sister, he gets the support from the Duke (specially since the sister is the favourite daughter) and a beautiful love story to show to high society. The sister is smart enough to raise her status and "defeat" the legitimate daughter, so it's not like she is a bad option to the prince. It's a win win situation for him, even without love.
And guys let's not forget that the mc in the previous life was a bitch who bullied her sister. Maybe she was goaded to do that by others but she still did it. She was spoilt, noisy and rude.