The Mo family seems like a typical old noble family. Far more interested in maintaining their public image, sticking to the old ways, and jealously guarding their phoney prestige than really trying to fulfill their mission. The real Xuanyuan Yu must have got sick of that sort of world, which is why she's behaving so rebelliously, to the point of leaving her own family, which resulted in Fourteen being sent instead, to her doom. It's pretty weird that for the Xuanyuan family sticking to the traditions and trying to suck up to the Mo family was actually worth the risk of sending someone who could be a demon, when the Mo family is a demon hunter family.