It stated somewhere that Cedric was never in love with the younger sister/first wife. He was attached to her, but not in a romantic way. They only got married to fulfill an old promise and to be named heir. Cedric only married her because he saw how malicious the Haley/Older sister was.

He was always meant to fall in love with Dillion. I think it confirms it when he was really focused on her rather than the younger sister when he went to propose at the beginning of the story. They were lucky to meet sooner in this new timeline rather than later. 

In the old timeline, if younger sister didn't die and met Dillion(in Adeline body) later, most likely a scandal would have happened. The relationship between both families would have been broken as well because Cedric would have done anything to be with Dillion even if that meant abandoning his wife.