Du Yunni and her younger brother were unwilling to call Cui Jueyan’s parents "Grandpa" and "Grandma." For one thing, they had little contact with the Cui family, lived apart, and rarely saw them even during holidays, so there was no real bond to speak of.
In their hearts, they had only ever had one grandfather: the father of Du Lianshuang and the great-uncle to Du Anrao and the others. He was the elder who had watched them grow up, the one who had truly loved and protected them. To them, the Cuis were merely strangers who happened to share a bloodline.
Secondly, it was, of course, because of the despicable things Cui Jueyan and the rest of the Cui family had done.
Chapter 386