I recall some novel spoilers that give more depth on Cedric's family issues, esp. his grandpa.
The grandpa didn't like Cedric's mom because she was a commoner, and always gave her shit over it. Although grandpa loved Cedric, there was always that hint of class bias towards him due to his commoner lineage (something which the uncle's family exploited in the hopes of them getting the family headship). Another reason grandpa wanted Cedric to marry the Langtons was because he thought the Langtons' pureblood nobility would help raise Cedric's own status and (somewhat) wash away his commoner status.