Because of their current age difference and his friend Jace Green von Imperlian, Yara von Chrysalis referred to the Grand Marshal as "Uncle." Because he is Jace Green's uncle, Yara addressed him as such. When their ages are compared, Yara von Chrysalis is in his teens, barely twenty years old, while the Grand Marshal is in his forties.
People in this world can live up to 200 years for ordinary humans and more than 200 years for awakened ones. There isn't much of a difference in age between 29 and 40. However, because Ye Xiajie and Hei Anjing's original relationship is that of husband and wife, being addressed as 'Uncle' by his beloved astounded the Grand Marshal.
Hearing the incarnation of Lord Hei refers to the Lord God Ye as 'Uncle' is quite shocking for Adjutant Kir von Giselle and Dean Frey von Gregory, who know their true identities. It's too amusing to hear and see such a scene. That's why Warden Lou almost burst out laughing when he heard Yara refer to their boss as Uncle.
