
He clearly liked her in the last life too. Why did he keep refusing her then?

Y'know it's completely possible to have romantic feelings for someone but not enough to actually want to do anything beyond that. For MC, his baggage preceded every decision in his life. Though he may have liked her in the past, I could understand not having the emotional capacity to give more than he could. Am I even making sense?
Same with present life. He wants to be with his family. His focus is entirely on them. His hungry for the time because he never had these moments with them. Living 'normally' seems absurd for one so young but he's lived a life of being destitute and orphaned. He doesn't have the time for anything else. He wants to just be THERE.
He had money. Had great business opportunity but he was just so very lonely ā˛ĨââŽā˛Ĩ I wonder now if his parents will push him for more opportunities like studying in the capital, encouraging him to work with the 'uncle,' and other stuff too.
I just want all of them to be happy.

Even in the 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.
I'm glad Blue hair came to his senses.
When he told her to follow her heart cuz she deserves it, I knew he truly meant it.