Dimo never liked his stepmother so he called him Aunt Sonia instead of "Mother".
His mother was irreplaceable to him so he did not want to call someone else as his mother. He still treated Sonia with respect though despite how she treated him. He also fully believed that Brent and Monica were his siblings.
But that night, Dimo also overheard that his supposed siblings were the head butler's children.
Sonia and the head butler already had a relationship even before she married Dimo's father. Sonia married Duke Olivier for their riches and properties.
And the two adults were planning to have another child hoping for another boy.
The head butler was his father's cousin so the resemblance was ignored. It could be said that Brent and Monica got it from the Duke instead of the head butler.
Thus, Dimo nor his father never suspected it. Moreover, the head butler had a wife of his own but she was apparently barren.
