
I think you might want to reread the novel. Cecil is their second son. But Josh enjoys it's when Cecil dresses like a girl bc Cecil is an exact replica of Chase when he was young and did the chocolate commercial. Cecil dresses as a girl to make Josh happy, to which Pete teases Cecil for being weird.

All Millers look like Ashley, who looks like Dominic. Cecil has blue eyes, Chase originally had blue eyes before he manifested. Grayson never had blue eyes.
And Cecil looks more like a Miller than Pete ( who is Josh's mini-me).
Grayson doesn't have children. And the only one he will have with in the future would be Dane.
In the other story Pete and Cecil didn’t seem to have that big of an age gap so maybe this is where Cecil is conceived?