Lucius was the second son? Cedric was son to heir?
No, Cedric is the son of the 7th Earl's youngest son, Charles. There was John, then Lucius, then Charles.
Back in part 4 it says that Lucius is Cedric's only living uncle.
In part 6, when Lucius is showing Lavinia and her father the gallery, and Lavinia is standing in front of the little picture of Cedric with his mum and dad, Lucius says, "Oh, yes, that's my youngest brother, Charles."
When John died without issue, Lucius became the heir. If Lucius dies without issue, Cedric, as his father's only male child, will inherit the Earldom.
Hope this helps with this question.
